Wednesday, June 12, 2024

Nicotine is hard to ban: Juul wins a reprieve from the FDA, and illegal vapes flood the market

 Here are two stories by Jennifer Maloney at the WSJ:

FDA Rescinds Juul Ban, Opening Door for Federal Clearance. E-cigarette maker’s products have stayed on market pending appeal of 2022 ban.. By  Jennifer Maloney

"The Food and Drug Administration rescinded its 2022 ban on Juul Labs’s e-cigarettes. The agency hasn’t yet reached a final determination on whether they can stay on the U.S. market, but the move opened the possibility for federal clearance.

The FDA in 2022 ordered Juul to halt its sales, then stayed the order pending the vaping company’s appeal. The agency said Thursday that it was placing Juul’s products back under scientific review, essentially moving them back to their regulatory status before the ban. 

...

"Juul’s products remain on the market. The FDA didn’t give a timeline for a final decision on whether they can stay there. Juul is the No. 2 e-cigarette maker in the U.S.

Juul and other e-cigarette manufacturers in 2020 were required to submit scientific research to demonstrate that their vaping products exposed users to fewer carcinogens than cigarettes and that the benefit of helping adult smokers switch to a safer alternative outweighed the potential harm of hooking young people on nicotine.

...

"The FDA ban, though it was quickly put on hold, sent Juul into a financial tailspin. The company narrowly averted bankruptcy. Juul has since submitted next-generation vaping products for FDA review. They aren’t yet for sale in the U.S."

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U.S. Pledges Crackdown on Illegal E-CigarettesFDA and DOJ form task force to go after fruity, disposable vapes flooding the market.  By Jennifer Maloney

"Big tobacco companies and their critics agree on at least one thing: The illegal, fruit-flavored, disposable vapes that are popular among teenagers have flooded the U.S. market and federal regulators haven’t done enough to stop it.

"The Food and Drug Administration and Justice Department said Monday they are stepping up enforcement by forming a multiagency task force to go after the illegal distribution and sale of e-cigarettes.

"Disposable vaping devices, almost none of which are authorized for sale by the FDA, represent more than 30% of U.S. e-cigarette sales in stores tracked by Nielsen, according to an analysis by Goldman Sachs. Many of them are imported from China. Breeze Pro and Elfbar, both of which were ordered off the market last year by the FDA, remain the top two disposable e-cigarette brands in the U.S.

"Njoy is the only disposable vaping brand authorized for sale by the FDA." 

Tuesday, June 11, 2024

Frans de Waal (1948-2024)

 The eminent primatologist Frans de Waal has passed away. Here's a memoriam from Emory University:

Emory primatologist Frans de Waal remembered for bringing apes ‘a little closer to humans’

I sometimes show the video below about his experiment with monkeys on fairness (and being treated unfairly) to my class on experimental economics (typically when I'm about to talk about the ultimatum game).

"Two capuchins were situated in enclosures next to one another. A researcher would ask them to do a task and if they succeeded give them a treat. The catch was one monkey was always rewarded with a piece of cucumber while the other monkey sometimes got a piece of cucumber and sometimes got a grape — a preferred treat among capuchin monkeys.

"A video de Waal filmed of one of the experiments created a media sensation.

Unequal pay for equal work: When the first monkey gives the researcher a rock, she is rewarded with a cucumber slice. But watch what happens when the first monkey sees the second monkey hand the researcher a rock — and get a much tastier grape instead.

"A monkey that received only cucumber appears perfectly happy until she sees her companion receive a grape. Then her behavior changes. She accepts the next piece of cucumber only to throw it back at the researcher, pounding the surface in front of the enclosure and shaking its Plexiglas walls.

“That video struck home with a lot of people,” Brosnan says. “Who hasn’t felt like that monkey that’s only getting cucumbers? Our research showed something about the evolution of the sense of human fairness.”



Monday, June 10, 2024

INFORMS Section on Auctions and Market Design (AMD)

 Itai Ashlagi and Vahideh Manshadi write:

"Dear colleagues:

 

We are writing to provide updates about the ongoing activities of the INFORMS Section on Auctions and Market Design (AMD)... 

 

1) AMD Membership without INFORMS Membership: As you may know, AMD aims to build an inclusive and diverse community interested in Market Design (broadly construed). Toward that goal, we have worked with INFORMS to create the option of joining AMD without INFORMS membership. If you are interested in joining the AMD section, please visit our website and check out the different options to join (as INFORMS member for an additional $10 on top of your INFORMS membership or non-member for only $20 in total per year). This way you will continue to be informed about our market design activities. 

 

2) Special Issue on Market Design (Deadline September 24): We are pleased to announce that we have co-sponsored a new Special Issue on Mathematics of Market Design at the INFORMS Journal Mathematics of Operations Research. (Special Issue Editors: Saša Pekeč, Martin Bichler, Nicole Immorlica, Scott Kominers, and Paul Milgrom) 

 

3) Journal Presence at Management Science: the INFORMS Journal  Management Science now has a department titled Market Design, Platform, and Demand Analytics. (Dept. Editors are Itai Ashlagi, Martin Bichler, and Srikanth Jagabathula; the list of AEs includes Paul Milgrom and Al Roth); Management Science is the flagship journal of INFORMS.  

 

4) AMD Workshop at the ACM EC in July 24: We are pleased to announce the INFORMS Market Design Workshop which will take place in conjunction with the ACM EC Conference at Yale University, July 8-11, 2024. Special thanks to Paul Dutting, John Horton, and Yash Kanoria for co-organizing the workshop. Check the workshop website for the program details.

 

5) AMD INFORMS Cluster at the Annual Meeting in October 24: We are excited to have organized  ~25 invited sessions on a wide range of topics as part of the AMD cluster at the upcoming INFORMS Annual Meeting (Seattle, Washington, October 20-23, 2024); special thanks to Thodoris Lykouris, Ali Makhdoumi, Pengyu Qian for serving as the cluster co-organizers.

 If you want to learn more about AMD, please check out the AMD website, people involved, and past activities. We'd be excited to have you as part of our growing community of market designers!"


Sunday, June 9, 2024

Recent kidney transplant papers

 Here are two new papers on kidney exchange that caught my eye, and one on incentivizing deceased donation by prioritizing registered donors on the deceased donor waiting list.


This one concerns organizing international kidney exchanges between countries while making sure that each one gets their fair share. (All exchanges are between 2 pairs.)

Benedek, Márton, Péter Biró, Daniel Paulusma, and Xin Ye. "Computing balanced solutions for large international kidney exchange schemes." Autonomous Agents and Multi-Agent Systems 38, no. 1 (2024): 1-41.

Abstract: To overcome incompatibility issues, kidney patients may swap their donors. In international kidney exchange programmes (IKEPs), countries merge their national patient–donor pools. We consider a recently introduced credit system. In each round, countries are given an initial “fair” allocation of the total number of kidney transplants. This allocation is adjusted by a credit function yielding a target allocation. The goal is to find a solution that approaches the target allocation as closely as possible, to ensure long-term stability of the international pool. As solutions, we use maximum matchings that lexicographically minimize the country deviations from the target allocation. We perform, for the first time, a computational study for a large number of countries. For the initial allocations we use two easy-to-compute solution concepts, the benefit value and the contribution value, and four classical but hard-to-compute concepts, the Shapley value, nucleolus, Banzhaf value and tau value. By using state-of-the-art software we show that the latter four concepts are now within reach for IKEPs of up to fifteen countries. Our experiments show that using lexicographically minimal maximum matchings instead of ones that only minimize the largest deviation from the target allocation (as previously done) may make an IKEP up to 54% more balanced.

"We consider IKEPs in the setting of European KEPs which are scheduled in rounds, typically once in every three months.

...

"We first note that the search for an optimal exchange scheme can be done in polynomial time for 2-way exchanges (matchings) but becomes NP-hard as soon as 3-way exchanges are permitted."

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Here's a paper that reports simulations on Using deceased donor kidneys to start living donor kidney exchange chains

Verma, Utkarsh, Nayaran Rangaraj, Viswanath Billa, and Deepa Usulumarty. "Long term simulation analysis of deceased donor initiated chains in kidney exchange programs." Health Systems (2023): 1-12.

ABSTRACT: Kidney exchange programs (KEPs) aim to find compatible kidneys for recipients with incompatible donors. Patients without a living donor depend upon deceased donor (DD) donations to get a kidney transplant. In India, a DD donates kidneys directly to a DD wait-list. The idea of initiating an exchange chain starting from a DD kidney is proposed in a few articles (and executed in Italy in 2018), but no mathematical formulation has been given for this merger. We have introduced an integer programming formulation that creates DD-initiated chains, considering both paired exchange registry and DD allocations simultaneously and addressing the overlap issue between the exchange registry and DD wait-list as recipients can register for both registries independently. A long-term simulation study is done to analyse the gain of these DD-initiated chains over time. It suggests that even with small numbers of DDs, these chains can significantly increase potential transplants.

#########

And here's the paper on incentivizing registration to be a deceased donor.

Li, Mengling, and Yohanes E. Riyanto. "Incentivizing Organ Donation Under Different Priority Rules: The Role of Information." Management Science (2024).

Abstract: This paper examines the incentive to register for deceased organ donation under alternative organ allocation priority rules, which may prioritize registered donors and/or patients with higher valuations for organ transplantation. Specifically, the donor priority rule grants higher priority on the organ waiting list to those who have previously registered as donors. The dual-incentive priority rules allocate organs based on donor status, followed by individual valuations within the same donor status, or vice versa. Both theoretical and experimental results suggest that the efficacy of the donor priority rule and the dual-incentive priority rules critically depends on the information environment. When organ transplantation valuations are unobservable prior to making donation decisions, the hybrid dual-incentive rules generate higher donation rates. In contrast, if valuations are observable, the dual-incentive priority rules create unbalanced incentives between high- and low-value agents, potentially undermining the efficacy of the hybrid dual-incentive rules in increasing overall donation rates.

Saturday, June 8, 2024

The ethics of field experiments in Economics, in the Financial Times

 The Financial Times has a column about the recent twitter (X) discussion concerning our paper Social Media and Job Market Success: A Field Experiment on Twitter, by Jingyi Qiu, Yan Chen, Alain Cohn, and Alvin E. Roth.

That twitter-up concerned whether it is ethical to do field experiments in economics, in which some argued that the benefits that might accrue to treated market participants may disadvantage untreated market participants, including those not involved in the experiment and from whom consent was not obtained. (The FT column has a paragraph in which Douglas MacKay*, a bio-ethicist from UNC considers ethical issues that might arise if the market "is a zero sum competition.") 

Here's the FT article, and the snippet that covers our paper.

When is it OK for economists to experiment on people? A recent study has raised ethical questions about research.  by Soumaya Keynes, Financial Times.

"While most economic debates are about as spicy as boiled potatoes, others generate a bit more heat. A recent stir fell into the second category, in response to a new study of junior academics angling for jobs in economics. 

...

"Alvin Roth, one of the authors of that experiment, says: “I can’t imagine economists thinking of a market as zero sum.” Perhaps a social media post could alert someone to a candidate so impressive that they persuade their university to make an extra position available. He points out that plenty of people share papers on social media, adding: “It seems to me that things that aren’t unethical to do shouldn’t be unethical to study to find out their effect.”

############

Here's my earlier post, of the paper, with some thoughts on the ethics of experimenting.

Thursday, May 23, 2024

########
*And here is an interesting looking bibliography, including some papers by economists.
Prepared and Managed by Emma Cohn and Douglas MacKay

Friday, June 7, 2024

Workshop in Honor of Elizabeth Hoffman, today at University of Iowa

Betsy Hoffman is celebrated in Iowa today.

Workshop in Honor of Elizabeth Hoffman Sponsored by the Clarence Tow Fund. Organizers: Yan Chen, David J. Cooper, Laura Razzolini, and Lise Vesterlund, The University of Iowa Tippie College of Business, Department of Economics, June 7th, 2024

Here is the tribute on the website (and below that is the program of the conference):

"Elizabeth Hoffman, Professor of Economics, Iowa State University, President Emerita, University of Colorado,Systems

"Elizabeth Hoffman has enjoyed an extraordinary career as a researcher, mentor, and administrator. She began her career as a historian, earning a PhD in history from the University of Pennsylvania in 1972 and working as an Assistant Professor at the University of Florida. Dr. Hoffman then returned to graduate school in economics, earning her PhD from the California Institute of Technology in 1979. This led to a distinguished career as an economics professor, working at Northwestern, Purdue, Wyoming, and Arizona.

"Dr. Hoffman’s research featured seminal contributions to experimental economics, law and economics, and cliometrics. The experiments that Dr. Hoffman conducted with Kevin McCabe, Keith Shachat, and Vernon Smith using double blind techniques to socially isolate subjects remain some of the most influential work ever conducted on other-regarding preferences, fundamentally changing how economists conceptualize other-regarding preferences. Her work with Matt Spitzer used insights from economic experiments to address fundamental issue in law and economics.

"In the late 90s, Dr. Hoffman began her work mentoring two groups of junior women economists through programs sponsored by the American Economic Association’s Committee on the Status of Women in the Economics Profession. This mentoring work was enormously influential, helping launch the careers of many prominent women economists (Bogan, Chen, Croson, Malmendier, Razzolini, Solnick, Vesterlund, and Washington). In large part due to her work as a mentor, Dr. Hoffman was awarded the American Economic Association’s 2010 Carolyn Shaw Bell Award for improving the status of women in the economics profession. It is a sign of Dr. Hoffman’s vast influence that three of her mentees have also won this award. Dr. Hoffman has served the profession in many other ways beyond mentorship. She served as the fourth president (and first woman president) of the Economic Science Association, the primary professional organization for experimental economics. She also served as a founding trustee for the Cliometric Society and the International Foundation for Research in Experimental Economics.

"Midway through her career, Dr. Hoffman shifted to a focus on administration. She served as Dean of Arts and Sciences at Iowa State (1993 – 1997), Provost at the University of Illinois, Chicago (1997 – 2000), President at the University of Colorado (2000 – 2005), and Provost at Iowa State (2005 – 2012). Since stepping down as provost, Dr. Hoffman has continued her productive career as a professor, producing numerous research articles and advising large numbers of PhD students."


Here's the program:

8:30 am – 9:00 am Welcomes and Introductions

Opening Remarks: David J. Cooper Tippie and Rollins Chair in Economics Henry B. Tippie College of Business, University of Iowa

9:00 am – 10:00 am Speaker: Catherine Eckel University Distinguished Professor in Dept of Economics Texas A&M University Paper Title: Dictator Games

10:00 am – 10:20 am Coffee Break

10:20 am – 10:50 am Speaker: Tony Kwasnica  Rob and Hope Brim Eminent Scholar Chair Florida State University Paper Title: Pennies from Heaven? Costly vs Free Bids in Penny Auctions

10:50 am – 11:20 am Speaker: Tanya Rosenblatt  Professor of Information and Economics University of Michigan  Paper Title: I've Got News for You!

11:20 am – 11:30 am Break

11:30 am – 12:00 pm Speaker: Yuanxiang Li Assistant Professor, Information Systems & Operations Management Soffolk University Paper Title: Designing an incentive mechanism for information security policy compliance: An experiment

12:00 pm – 12:30 pm Speaker: Yan Chen  Daniel Kahneman Collegiate Professor of Information School of Information University of Michigan  Paper Title: Social Media and Job Market Success: A Field Experiment on Twitter

12:30 pm – 1:30 pm Lunch

1:30 pm – 2:00 pm Speaker: Tom Rietz  Soumyo Sarkar Professor in Finance  Tippie College of Business, University of Iowa  Paper Title: Peering into the Black Box: Trader Strategies in the Iowa Electronic Markets

2:00 pm – 2:30 pm Speaker: Laura Razzolini  Department Head of Economics, Finance, and Legal Studies Professor of Economics Culverhouse College of Business, University of Alabama Paper Title: What have I learned from the Dictator Game over the years

2:30 pm – 2:40 pm Break

2:40 pm – 3:10 pm Speaker: Lise Vesterlund  Andrew W. Mellon Professor of Economics  University of Pittsburgh Paper Title: The Effect of Experimenter-Demand on Inference Across Populations

3:10 pm – 3:40 pm Speaker: Kevin McCabe Professor of Economics and Law  George Mason University Paper Title: An agent-based model of path-dependent market formation

3:40 pm – 4:00 pm Coffee Break

4:00 pm – 5:00 pm Speaker: Tom Palfrey Flintridge Foundation Professor of Economics and Political Science California Institute of Technology  Paper Title: Team Equilibrium: A General Theory of Team Games with an Application to Crisis Bargaining

5:30 pm – 6:30 pm Cocktail Hour and Betsy Hoffman Tribute Video

(The website also contains bios of each of the speakers).

HT: thanks to David Cooper for the link...

Thursday, June 6, 2024

Medical aid in dying debated in NY, extended in the Netherlands

 Here are two related stories, one from the NYT about a debate on whether to allow medical aid in dying in New York State. The other is from Fox News, about the medically assisted death of a physically healthy young woman afflicted with a frightful psychiatric disorder.

In New York, there's a debate about whether to become the 11th state to legalize medical aid in dying.  Some of the opponents worry about a slippery slope, leading to the Netherlands, where mental illnesses can qualify candidates for such aid.

Here's the NYT on the debate in NY:

Doctor-Assisted Death Is Legal in 10 States. Could New York Be No. 11? Activists have renewed attention on legislation related to the emotional issue of so-called medical aid in dying that has long languished in Albany.

"New York is one of 19 states where lawmakers are considering bills that would legalize medical aid in dying, a practice that is legal in 10 states and Washington, D.C.

"The bill in New York would allow mentally competent, terminally ill adults with no more than six months to live to request prescriptions from their doctors for life-ending medication. The patients would have to be able to ingest the medication on their own, and only the person seeking to die could request the prescription.

...

"Opponents worry that some patients might choose to end their lives based on an inaccurate prognosis or after being pressured to do so. And while the current bill is restricted to terminally ill people, they worry that lawmakers could expand eligibility for medical aid in dying after any initial legislation is passed."

*************

Here's Fox News on the Netherlands:

Physically healthy Dutch woman dies by assisted suicide at age 29. Zoraya ter Beek died by assisted suicide in the Netherlands last week.  By Kendall Tietz Fox News,  June 1, 2024

"29-year-old Zoraya ter Beek's life was terminated last week after waiting three years for final approval for her euthanasia, which is legal in the Netherlands if the patient is deemed to be experiencing "unbearable suffering with no prospect of improvement."

...

"she tried various things to treat her mental illness, including 33 rounds of electroconvulsive therapy, in which electric currents jolt the brain. But, after her last treatment in August 2020, her psychiatrist told her, "There’s nothing more we can do for you. It’s never going to get any better."

...

"My whole friends and my support system, we really did it together," she had told The Free Press. Ter Beek reportedly saw herself as an ambassador for the Dutch euthanasia program and believed there is proper protocol in place to prevent abuse of the system. 

"We’ve had this law for more than 20 years," she had told the outlet. "There are really strict rules, and it’s really safe."

Wednesday, June 5, 2024

Paying college athletes before it was legal

 Yesterday I blogged about the new NCAA rules on allowing college athletes to be paid: The ban on paying college athletes is history

It will no doubt shock you to learn that payments were made long before they were legal.  

The Guardian has a book review about that:

Hot Dog Money: behind the bribery scandal that rocked college basketball. A new book looks back at the federal investigation that found bribery and corruption within a major industry, by Andrew Lawrence, Tue 4 Jun 2024

"On 26 September 2018, 10 prominent US college sports figures were arrested in connection with a federal investigation into fraud and corruption. Specifically, the government alleged that business managers and financial advisers had bribed basketball coaches to secure business with NBA-bound players, and that a senior executive with Adidas had further conspired with them to funnel payments to high school players and their families in exchange for their commitment to Adidas-sponsored college sports programs.

"The scandal – which ensnared the top NBA draft pick Deandre Ayton, hall of fame coach Rick Pitino and Kobie Baker, the associate athletic director at Alabama, one of the country’s premier talent factories – was a black eye for the NCAA, the keystone cops who style themselves as virtuous defenders of amateurism in college sports while reaping billions off the backs of student-athletes, the majority of them Black and quite economically disadvantaged. The extent of the scheme wasn’t fully understood until one of the schemers, a middle-aged moneyman named Marty Blazer, was turned into an FBI informant. 

...

"Lawson’s latest nonfiction book, Hot Dog Money, is Blazer’s Goodfellas story – one largely told from Lawson’s one-on-one interviews with Blazer

...

"He was a mid-level financial adviser making six figures trading stocks and bonds with a client roster that slowly grew to include select members of the NFL’s Pittsburgh Steelers. The story of William “Tank” Black, the powerful football agent indicted for running a Ponzi scheme fueled with Detroit coke money, sparked Blazer’s larger ambitions.

"Blazer teamed up with an agent and recruited football players from Pennsylvania colleges with the aim of attaching himself to future pros. That’s where the “hot dog money” came in. Blazer didn’t just pass cash-filled envelopes under the table. He sent money home to players’ struggling families, supplied them with luxury cars, paid for lavish trips to Miami and Las Vegas, and comped their inevitable strip club binges. Sometimes he’d arrange to have girls flown in for parties closer to campus. “The girls are being trafficked, the kids are being trafficked,” says Lawson. “Forget morality, how do you even describe the decency of it all? This is what the swirling of a flushing toilet looks like.”

"In a typical hot dog money scheme, a college player receives cash in the form of a forgivable loan with the understanding that the bagman’s aboveboard services will be retained once the player turns pro; depending on the player, the bagman can make his money back many times over in boring management fees. A proudly devoted husband and father of three, Blazer was more interested in helping his clients make the most out of a corrupt system and went the extra mile to look out for them, paying for information that could help clients avert potential disaster.


Tuesday, June 4, 2024

The ban on paying college athletes is history

 The idea that paying college athletes is wrong has given way to the realities of the sports markets in which they perform.  For many years, college athletes were required to be unpaid amateurs, but that time has passed.

The WSJ has the story.

NCAA Agrees to Share Revenue With Athletes in Landmark $2.8 Billion Settlement. Breaking with more than a century of policy, the NCAA will pay billions in damages to former athletes and allow schools to pay athletes up to $20 million a year    By Laine Higgins  and Jared Diamond

"The National Collegiate Athletic Association and the five most prominent athletic conferences agreed to a $2.77 billion settlement of a class-action lawsuit on Thursday, ushering in a new era of college sports in which schools can pay athletes directly. 

"The move marks a dramatic shift for the NCAA, breaking with its century-old stance that college athletes are amateurs and therefore cannot share in any of the money they generate for their universities.

...

"It also marks the latest rule the NCAA has been forced to change amid an onslaught of legal challenges in recent years. 

"First, the NCAA allowed athletes to receive academic bonuses and profit from their name, image and likeness. Now, the biggest domino of all has fallen: For the first time ever, some players are going to be paid directly by their schools for playing their sports—a seismic shift that will completely reshape the business model for the top end of this billion-dollar industry. 

"The result is the creation of a system that will give Division I schools the ability to distribute roughly $20 million a year to their athletes, said people familiar with the matter. "


Monday, June 3, 2024

Kidney exchange between Israel and the Czech Republic

 The Jerusalem Post reports another kidney exchange between Israel and the Czech Republic:

Miraculous kidney donation between friends spans continents and save lives. The complex and sensitive cross-continental operation required coordination between senior officials in both countries.   https://www.jpost.com/international/article-804396   By JERUSALEM POST STAFF  MAY 30, 2024 18:39

"it was soon discovered that Glaor was not a direct match for a transplant, and their medical details were entered into the National Transplant Center's cross-matching database. The entry resulted in an international kidney transplant exchange in which Glaor donated his kidney to a patient in the Czech Republic, and the wife of the Czech patient donated her kidney to Shitrit.

"The complex and sensitive cross-continental operation required coordination between senior officials in both countries, with an emphasis on the precise timing of operating rooms, flights, and quick security and customs arrangements.

"The operation began in the middle of the night at the Hadassah University Medical Center in Jerusalem’s Ein Kerem. It was managed and coordinated by the National Transplant Center.

"Dr. Tamar Ashkenazi, director of the National Transplant Center, was in charge of coordinating the transplant.  Alongside her on the flight to Prague were coolers containing two kidneys that were removed overnight from two Israeli donors at Hadassah Ein Kerem, which were intended for transplantation in two Czech patients. 

"In the evening, at around 10 p.m., the transplants in Israel were completed, and the Czech kidneys successfully functioned in the bodies of the Israeli recipients, as reported similarly from Prague regarding the Israeli kidneys that were transplanted in the Czech Republic.

""We planned the process precisely, and were in constant contact with our counterparts in the Czech Republic regarding the surgical issues. Every action that takes place is critical and impacts the continuation of the operation," said Dr. Ashraf Imam, a senior surgeon in the transplant unit at Hadassah, who participated in the transplant operation."


Sunday, June 2, 2024

Kidney Exchange in Latin America and the Caribbean

 Kidney exchange isn't yet thriving in Latin America, but the basic infrastructure is in place. It would make a lot of sense to jumpstart kidney exchange by allowing cross border exchange, so that there would be a large enough pool of patient-donor pairs to make finding a match easy. Here's an article surveying the member countries of the Latin America and Caribbean Transplant Society.

Bastos, Juliana, David José de Barros Machado, Raquel Megale Moreira, Gustavo Fernandes Ferreira, and Elias David-Neto. "Kidney Paired Donation in Latin America and the Caribbean: An Update." Transplantation 108, no. 6 (2024): 1257-1258.

"we assess the situation of KPD in the countries affiliated with the Latin America and Caribbean Transplant Society (STALYC).

  

"Guatemala was the first country to publish a scientific report on KPD in 2018,8 with 4 kidney paired transplants performed between 2010 and 2017.

"Two reports from Argentina on local news websites reported 2-way exchanges involving 2 pairs in 20159 and 2018.10

"Costa Rica published a 2-way exchange on the hospital’s social media page in 2016,11 whereas a 2-way exchange transplantation was performed in Brazil in 2020.12

"Mexico is leading the reported KPD activity with a first experience involving 4 pairs in a chain beginning with an altruistic donor13 reported in 2019. A more recent publication reported on 22 pairs transplanted with longer chains and excellent results.14

"It is interesting to note that there are 6 countries—Panama, Ecuador, Venezuela, Peru, Chile, and Paraguay—with laws explicitly permitting KPD. Thus far, there has not been a report on KPD in those countries, which is likely due to the relatively recent publication of these legislations, all of which occurred after 2010.

...

"A recent publication has shed light on the potential of KPD in low-to-middle income countries (LMICs), strongly advocating for the promotion and encouragement of KPD programs, including considerations of cost advantages.17 Of additional relevance, valuable recommendations on initiating KPD programs in LMICs include starting with smaller chains, considering simultaneous surgeries, and implementing effective organ transport strategies.17 By adopting these strategies, LMICs can address compatibility issues and enhance their organ transplantation capabilities.

"Considering that KT is the superior and more cost-effective treatment option for patients with CKD, it is puzzling that the initiation of KPD programs remains limited in a region primarily composed of LMICs. Although some countries may have implemented KPD programs without publication, genuinely active programs beyond Mexico remain missing. It is crucial to emphasize that in most of these countries, deceased donor transplantation also falls significantly short of estimated needs.3,7 The entire infrastructure surrounding transplantation, including both living and deceased donors, continues to require substantial improvements. Particularly for KPD, initiatives such as educational campaigns for physicians, recipients, and donors, as well as investments in logistics and software in addition to a legal framework, need to be encouraged. Similar to KPD programs in Europe,5,18 collaborative efforts across countries could benefit smaller countries. Transplant societies, including STALYC, could play a vital role in supporting the advancement of paired donation, ensuring improved access to transplantation for their populations, especially with living donors."

Saturday, June 1, 2024

The Path to a Match for Interventional Cardiology Fellowships

The Society for Cardiovascular Angiography & Interventions has started a fellowship match, and here's an article describing the familiar marketplace failure that led to that decision, involving unraveling of application, interview and appointment dates, with the resulting congestion and exploding offers, and the process of reaching sufficient consensus to move to a centralized match ( to be run by the NRMP).

The Path to a Match for Interventional Cardiology Fellowship: A Major SCAI Initiative  by Douglas E. Drachman MD, FSCAI (Chair) a, Tayo Addo MD b, Robert J. Applegate MD, MSCAI c, Robert C. Bartel MSc, CAE d, Anna E. Bortnick MD, PhD, MSc, FSCAI e, Francesca M. Dea d, Tarek Helmy MD, MSCAI f, Timothy D. Henry MD, MSCAI g, Adnan Khalif MD, FSCAI h, Ajay J. Kirtane MD, SM, FSCAI i, Michael Levy MD, MPH, FSCAI j, Michael J. Lim MD, MSCAI k, Ehtisham Mahmud MD, MSCAI l, Nino Mihatov MD, FSCAI m, Sahil A. Parikh MD, FSCAI i, Laura Porter CMP d, Abhiram Prasad MD n, Sunil V. Rao MD, FSCAI o, Louai Razzouk MD, MPH, FSCAI o, Samit Shah MD, PhD, FSCAI p, Adhir Shroff MD, MPH, FSCAI q, Jacqueline E. Tamis-Holland MD, FSCAI r, Poonam Velagapudi MD, FSCAI s, Fredrick G. Welt MD, FSCAI t, J. Dawn Abbott MD, FSCAI (Co-Chair), Journal of the Society for Cardiovascular Angiography & Interventions, in press.

"Abstract: The field of interventional cardiology (IC) has evolved dramatically over the past 40 years. Training and certification in IC have kept pace, with the development of accredited IC fellowship training programs, training statements, and subspecialty board certification. The application process, however, remained fragmented with lack of a universal process or time frame. In recent years, growing competition among training programs for the strongest candidates resulted in time-limited offers and high-pressure situations that disadvantaged candidates. A grassroots effort was recently undertaken by a Society for Cardiovascular Angiography & Interventions task force, to create equity in the system by establishing a national Match for IC fellowship. This manuscript explores the rationale, process, and implications of this endeavor."


"over the past several years program directors and candidates found that the process has devolved, with wide variation in application timelines and on-the-spot offers, which disadvantage candidates and programs looking to interview a range of applicants.

"The pressures and unfair features of the existing system were further fueled by the transition to virtual interviews related to the COVID-19 pandemic. With logistics of travel no longer a consideration, programs could commence interviews nearly immediately after the applications became available. This led to more candidates being interviewed in rapid succession, and a system evolved in which programs quickly assessed candidates, offered positions, and applied pressure for candidates to accept offers or be passed over for other candidates.

"In response to the shortcomings of the current system, members of Society for Cardiovascular Angiography & Interventions (SCAI) were inspired to lead a grassroots educational campaign to organize IC program directors and the broader interventional community to commit to a regulated “Match” process under the established National Resident Match Program (NRMP). This manuscript provides an account of how this process unfolded and how a Match for IC fellowship was ultimately created.

...

"From the applicant’s perspective, the lack of a structured timeline for the application process required candidates to make career decisions early in the first year of cardiovascular disease training and to compose their application materials 2 years in advance of starting IC training. With ERAS open to application submission in the fall of the second year for the December release to programs, fellows had limited time on clinical rotations to determine their interest and aptitude for IC. Additionally, letters of recommendation, written at this early stage, risked not being fully reflective of each candidate’s capacity to improve and develop the technical skills and clinical knowledge important for success in the field. There were other disadvantages to candidates in the existing system. Fellows at programs with an IC fellowship had an advantage of securing an internal spot but were often pressured to limit their exploration of the opportunities at other programs, potentially disadvantaging them in the long term.

"Another problem with the existing system was that the pressure to recruit candidates on a tight timeline limited the opportunity to interview applicants from a wide variety and diversity of programs, potentially reducing the ability to recruit underrepresented candidates from varied programs. Despite an overall increase in the diversity of physicians entering the workforce,11 there has been little change in the applicant pool for IC over the years, with fewer than 5% of applicants self-reporting as Black race or Hispanic ethnicity and only 10% identifying as women.12

"Competition among the programs, each vying for the seemingly strongest candidates, degenerated into a system that favored quick decision-making on the part of programs to offer positions as early as possible. The influence of the COVID-19 pandemic in 2020 and 2021 negatively impacted an already high-pressure application process, compounding its many weaknesses.13 Fellowship interviews were hosted virtually rather than in person, which enabled candidates to interview at a greater number of programs without the need to travel. In addition, the virtual format accelerated the tempo of an application process that was already felt to be too fast, resulting in an increase in so-called “exploding offers”—offers that required the accepted candidate to respond within a very short timeframe or risk losing the offer. This practice placed significant pressure on candidates to make quick decisions, often forcing them to determine whether to accept the offer from 1 institution before having the opportunity to participate in interviews with—let alone see and evaluate—other programs or fully understand the ramifications of accepting an offer on their personal lives. At the same time, the accelerated timetable left many programs scrambling to identify applicants, as the number of available candidates diminished rapidly due to applicants accepting time-sensitive, exploding offers.

...

"As with other national efforts of this magnitude, the path to develop consensus in favor of a Match was not without challenges. There were several program directors around the country who strongly opposed the institution of a Match. These were well-regarded academicians and clinician educators who expressed very sincere concerns about the impact on fellows in their programs. The members of the SCAI Match Task Force addressed as many concerns as possible, providing the information necessary for each program director to make the best decision for their institution. A minority of program directors remained opposed to the initiative or did not engage with Task Force members despite multiple attempts to be contacted.

"The Match campaign proved highly effective, and by November 2022, the 75% threshold of programs and positions to implement the Match was met

...

"As the sponsor of the Match, SCAI considered the pros and cons of the “All In Policy,” where registered programs must attempt to fill all ACGME positions at the program through the Match.15,16 SCAI opted out of the “All In Policy” to allow programs to have flexibility for unique situations that require commitment to a candidate outside of the Match. 

...

"As a result of the successful implementation of the Match in IC, the first Match cycle for incoming IC fellows will open in the summer of 2024. Individuals eligible to apply include cardiovascular disease fellows in their third or final year of training and graduates who have completed fellowship and are in clinical practice. This class will start IC training in July 2025"



Friday, May 31, 2024

Organs, tissues, and medical devices are regulated very differently

 Here's an article in JAMA by a Michigan doctor and his Congresswoman, on the lack of regulation for some substances of human origin.

Urgent Need for Regulatory Oversight of Human Cells, Tissues, and Cellular and Tissue–Based Products, by Robert P. Dickson, MD; Deborah A. Dingell, MS, JAMA. 2024;331(20):1703-1704. doi:10.1001/jama.2024.6834

" In 2021, an outbreak of M tuberculosis occurred in the US when contaminated bone graft material was implanted into 113 patients, 77% of whom developed clinically apparent tuberculosis.

...

[Again in 2023] "the Centers for Disease Control and Prevention (CDC) had identified additional tuberculosis-infected patients who had received implants of the same product, harvested from the same donor. At final count, 36 patients in 7 states had undergone implantation of bone graft tissue contaminated with M tuberculosis.

...

"Given the rigorous safety testing required of most medical therapies, how could 2 lethal outbreaks of tuberculosis occur in as many years, arising from the same product, distributed by the same company?

"The answer lies in the FDA’s designation of this product as a human cells, tissues, and cellular and tissue–based product (HCT/P) (Table). This class of therapies, which includes bone grafts, skin grafts, and stem cells, is not subject to the same regulatory standards as pharmaceuticals, biological products (such as blood products and monoclonal antibodies), or organ transplants. This designation has profound regulatory and clinical consequences



Thursday, May 30, 2024

Sigecom Test of Time Award 2024 for AdWords and generalized online matching by Mehta, Saberi, Vazirani, and Vazirani

 The SIGecom Test of Time Award recognizes the author or authors of an influential paper or series of papers published between ten and twenty-five years ago that has significantly impacted research or applications exemplifying the interplay of economics and computation. More details and nomination procedure…

The Test of Time Award Winners for 2024 are Aranyak MehtaAmin SaberiUmesh Vazirani, and Vijay Vazirani  

They are cited for "introducing and solving a model of online matching with budgets that has seen many practical applications to online markets and broad and continuing impact in the literature."

in their paper

AdWords and generalized online matching, Journal of the ACM 54(5), 2007, Article 22

Abstract: How does a search engine company decide what ads to display with each query so as to maximize its revenue? This turns out to be a generalization of the online bipartite matching problem. We introduce the notion of a trade-off revealing LP and use it to derive an optimal algorithm achieving a competitive ratio of 1−1/e for this problem.

From the introduction:

"Internet search engine companies, such as Google, Yahoo and MSN, have revolutionized not only the use of the Internet by individuals but also the way businesses advertise to consumers. Typical search engine queries are short and reveal a great deal of information about user preferences. This gives search engine companies a unique opportunity to display highly targeted ads to the user.

"The online advertising mechanisms used by search engines, including Google’s AdWords, are essentially large auctions where businesses place bids for individual keywords, together with limits specifying their maximum daily budget. The search engine company earns revenue when it displays their ads in response to a relevant search query (if the user actually clicks on the ad). Indeed, most of the revenues of search engine companies are derived in this manner [Battelle 2005]. One factor in their dramatic success is that, unlike conventional advertising, search engine companies are able to cater to low-budget advertisers (who occupy the fat tail of the power law distribution governing advertising budgets of companies and organizations).

"The following computational problem, which we call the adwords problem, is a formalization of a question posed to us by M. Henzinger: There are (private communication, 2004). N bidders, each with a specified daily budget bi . Q is a set of query words. Each bidder i specifies a bid ciq for query word q ∈ Q. A sequence q1q2 · · · q M of query words q j ∈ Q arrive online during the day, and each query q j must be assigned to some bidder i (for a revenue of ciq j ). The objective is to maximize the total revenue at the end of the day while respecting the daily budgets of the bidders.

"In this article, we present a deterministic algorithm achieving a competitive ratio of 1 − 1/e for this problem, under the assumption that bids are small compared to budgets."

###########

Last year's award:

Monday, April 3, 2023

Test of Time Award 2023 to Immorlica & Mahdian, and Ashlagi, Kanoria & Leshno

#######

"Past and Present Members of the Test of Time Award Committee: Yeon-Koo Che, Yiling Chen, Nikhil Devanur, Joan Feigenbaum, Jason Hartline, Bobby Kleinberg, Paul Milgrom, Noam Nisan, Asu Ozdaglar, David Parkes, David Pennock, Alvin Roth, Tim Roughgarden, Larry Samuelson, Tuomas Sandholm, Yoav Shoham, Éva Tardos, Moshe Tennenholtz"


Wednesday, May 29, 2024

Marijuana policy and use in the U.S., 1979-2022, by Jonathan Caulkins, in Addiction

 Here's a paper forthcoming in the journal Addiction:

Changes in self-reported cannabis use in the United States from 1979 to 2022, by Jonathan P. Caulkins, published online 22 May 2024, https://doi.org/10.1111/add.16519 

"Abstract

Background and aims: Multiple countries are considering revising cannabis policies. This study aimed to measure long-term trends in cannabis use in the United States and compare them with alcohol use.

Design and setting: Secondary analysis of United States general population survey data.

Participants: The national surveys had a total of 1 641 041 participants across 27 surveys from 1979 to 2022.

Measurements: Rates of use reported to the US National Survey on Drug Use and Health and its predecessors are described, as are trends in days of use reported. Four milepost years are contrasted: 1979 (first available data and end of relatively liberal policies of the 1970s), 1992 (end of 12 years of conservative Reagan-Bush era policies), 2008 (last year before the Justice Department signaled explicit federal non-interference with state-level legalizations) and 2022 (most recent data available).

Findings: Reported cannabis use declined to a nadir in 1992, with partial recovery through 2008, and substantial increases since then, particularly for measures of more intensive use. Between 2008 and 2022, the per capita rate of reporting past-year use increased by 120%, and days of use reported per capita increased by 218% (in absolute terms from the annual equivalent of 2.3 to 8.1 billion days per year). From 1992 to 2022, there was a 15-fold increase in the per capita rate of reporting daily or near daily use. Whereas the 1992 survey recorded 10 times as many daily or near daily alcohol as cannabis users (8.9 vs. 0.9 M), the 2022 survey, for the first time, recorded more daily and near daily users of cannabis than alcohol (17.7 vs. 14.7 M). Far more people drink, but high-frequency drinking is less common. In 2022, the median drinker reported drinking on 4–5 days in the past month, versus 15–16 days in the past month for cannabis. In 2022, past-month cannabis consumers were almost four times as likely to report daily or near daily use (42.3% vs. 10.9%) and 7.4 times more likely to report daily use (28.2% vs. 3.8%).

ConclusionsLong-term trends in cannabis use in the United States parallel corresponding changes in cannabis policy, with declines during periods of greater restriction and growth during periods of policy liberalization. A growing share of cannabis consumers report daily or near daily use, and their numbers now exceed the number of daily and near daily drinkers."

Daily and Near Daily (DND) use

...

"That is still not as high as for cigarettes. The 2022 NSDUH survey finds that 58.7% of PM ["Past Month"] cigarette smokers smoked ‘daily’—defined as ‘smoked one or more packs of cigarettes per day’ [8]. Therefore, there are more daily cigarette smokers than DND PM marijuana users (24.1 vs 17.7 million). 3 Still, patterns of marijuana consumption have shifted from being like alcohol to being closer to cigarette use. It is also no longer a young person's drug. In 2022, people 35 and older accounted for (slightly) more days of use than did those under the age of 35."

Tuesday, May 28, 2024

Gonzalo Arrieta defends his dissertation

Gonzalo Arrieta defended his dissertation last week.



Here's his job market paper:

Procedural Decision-Making In The Face OfComplexity by Gonzalo Arrieta and Kirby Nielsen

Abstract: A large body of work documents that complexity affects individuals’ choices, but the literature has remained mostly agnostic about why. We provide direct evidence that individuals use different choice processes for complex and simple decisions. We hypothesize that individuals resort to “procedures”—cognitively simpler choice processes that we characterize as being easier to describe to another person—as the complexity of the decision environment increases. We test our hypothesis using two experiments, one with choices over lotteries and one with choices over charities. We exogenously vary the complexity of the decision environment and measure the describability of choice processes by how well another individual can replicate the decision-maker’s choices given the decision-maker’s description of how they chose. We find strong support for our hypothesis: Both of our experiments show that individuals’ choice processes are more describable in complex choice environments, which we interpret as evidence that decision-making becomes more procedural as complexity increases. We show that procedural decision-makers choose more consistently and exhibit fewer dominance violations, though we remain agnostic about the causal effect of procedures on decision quality. Additional secondary evidence suggests that procedural decision-making is a choice simplification that reduces the cognitive costs of decision-making."

##########
Another of his papers is a really creative investigation of human welfare:

Abstract: The dominant approach to welfare is based on revealed preferences and thus is restricted to settings where the individual knows their preferences have been fulfilled. We use a choosing-for-others framework to experimentally study welfare when what the individual believes to be true differs from what is actually true. About 42% of participants see welfare as independent of beliefs; 22% see welfare as only depending on beliefs; and 29% see a lower, but still positive, welfare effect when beliefs are fixed. Furthermore, the average participant values accurate beliefs. Our results suggest most people support the idea that welfare goes beyond awareness, which can inform media regulation, informational policies, and government communication.

 

Here's a figure from the instructions about the creation of "real" and "fake" inscribed copies of books. A third party judged the welfare to the recipient

"Our altruistically revealed preference paradigm consists of asking surrogate participants to trade off a monetary bonus given to the Receiver, and the Receiver getting the books with the original notes over those with the fake notes. The bonus amount is a surprise to the Receiver to minimize concerns that they use it to deduce which books they got (i.e., to maintain obliviousness). Our three requirements allow us to interpret the bonus amount that leaves participants indifferent between giving the original and fake notes as a measure of the change in the Receiver’s welfare. As a benchmark, we also elicit the welfare effect when the Receiver does learn which notes they get."
********
Welcome to the club, Gonzalo.



Monday, May 27, 2024

Matching for love or profit at Stanford

 Looking for a date, a marriage, a business partner?  Stanford might have the right app for you.

Here's the story from the Stanford Daily

Match, Marry, Capitalize? A catalog of Stanford’s matchmaking services, By Oriana Riley

"Here’s how matchmakers have attempted to help the Stanford community find love, friendships and business partners. 

Marriage Pact 

Perhaps the most famous Stanford matchmaking service, Marriage Pact, originated as a final project for a Stanford economics project. The annual service, which uses a survey to match compatible individuals for friendships and romantic relationships, has expanded to 88 campuses and more than 400,000 people.

...

"Founder Pact 

"Founder Pact presented an opportunity for students to try their hand not at love, but entrepreneurship. Founder Pact, created by the Business Association of Stanford Entrepreneurial Students (BASES), aimed to match entrepreneurs to realize their business ideas together. 

"The Founder Pact form, however, is now closed. 

"Wing

"A baby bird on the Stanford dating scene is Wing, announced to Stanford students via email on April 18. According to the email, Wing is built on the idea of “set[ting] up your friends.” 

Sunday, May 26, 2024

Recent papers on matching: May GEB

 The May issue of Games and Economic Behavior has several papers on matching that caught my eye:

Games and Economic Behavior, Volume 145, May 2024

Strong core and Pareto-optimality in the multiple partners matching problem under lexicographic preference domains, by Péter Biró and Gergely Csáji

Abstract: We study strong core and Pareto-optimal solutions for multiple partners matching problem under lexicographic preference domains from a computational point of view. The restriction to the two-sided case is called stable many-to-many matching problem and the general one-sided case is called stable fixtures problem. We provide an example to show that the strong core can be empty even for many-to-many problems, and that deciding the non-emptiness of the strong core is NP-hard. On the positive side, we give efficient algorithms for finding a near feasible strong core solution and for finding a fractional matching in the strong core of fractional matchings. In contrast with the NP-hardness result for the stable fixtures problem, we show that finding a maximum size matching that is Pareto-optimal can be done efficiently for many-to-many problems. Finally, we show that for reverse-lexicographic preferences the strong core is always non-empty in the many-to-many case.


Bayesian stable states, by Yi-Chun Chen and Gaoji Hu 

Abstract: This paper extends the Bayesian stability notion of Liu (2020) to define the Bayesian stability of a market state, which consists of a matching outcome and an information structure. The information structure can be arbitrarily heterogeneous and can accommodate learning among agents. We first establish that a Bayesian stable matching function of Liu (2020) can be recast as Bayesian stable market states with homogeneous information. We then illustrate the usefulness of such an extension by (i) refining Liu's Bayesian efficiency notion to define the Bayesian efficiency of a market state and (ii) generalizing his result—that Bayesian stable matching functions are Bayesian efficient—to an analogous one for market states. More importantly, we show that (iii) a decentralized matching process converges to a Bayesian stable market state and thereby offer a decentralized foundation for Liu's Bayesian stable matching function.


Efficient matching under general constraints  by Kenzo Imamura, Yasushi Kawase

Abstract: We study indivisible goods allocation problems under constraints and provide algorithms to check whether a given matching is Pareto efficient. We first show that the serial dictatorship algorithm can be used to check Pareto efficiency if the constraints are matroid. To prove this, we develop a generalized top trading cycles algorithm. Moreover, we show that the matroid structure is necessary for obtaining all Pareto efficient matchings by the serial dictatorship algorithm. Second, we provide an extension of the serial dictatorship algorithm to check Pareto efficiency under general constraints. As an application of our results to prioritized allocations, we discuss Pareto improving the deferred acceptance algorithm.


Saturday, May 25, 2024

Size is important in liver exchange

 Liver exchange has a lot in common with kidney exchange, in the sense that the issues involved in forming cycles and chains once you know which donors are compatible with which patients are very similar.  But a big difference is what constitutes a compatible donor: for livers, size (of the donor, and the donor liver) is very important, sensitively so.

Here's a paper forthcoming in the American Journal of Transplantation, by a team of transplant physicians and economists (with kidney exchange experience), on the importance of size.

"Enhanced Role of Multi-Pair Donor Swaps in Response to Size Incompatibility: The First Two 5-Way and the First 6-Way Liver Paired Exchanges" by Sezai Yilmaz, MD, FACS, Tayfun Sönmez, PhD, M. Utku Ünver, PhD, Volkan Ince, MD, Sami Akbulut, MD, PhD, FACS, Kemal Baris Sarici, MD, and Burak Isik, MD, American Journal of Transplantation, Brief communication, in press.

Abstract: A significant portion of liver transplantations in many countries is conducted via living-donor liver transplantation (LDLT). However, numerous potential donors are unable to donate to their intended recipients due to factors such as blood-type incompatibility or size incompatibility. Despite this, an incompatible donor for one recipient may still be a viable donor for another patient. In recent decades, several transplant centers have introduced liver paired exchange (LPE) programs, facilitating donor exchanges between patients and their incompatible donors, thereby enabling compatible transplants. Initially, LPE programs in Asia primarily involved ABO-i pairs, resulting in 2-way exchanges mainly between blood-type A and B recipients and donors. This practice has led to a modest 1-2% increase in LDLTs at some centers. Incorporating size incompatibility alongside blood-type incompatibility further enhances the efficacy and significance of multiple-pair LPEs. Launched in July 2022, a single-center LPE program established at Inönü University Liver Transplant Institute in Malatya, Türkiye, has conducted thirteen 2-way, nine 3-way, four 4-way, two 5-way, and one 6-way LPEs until February 2024. In 2023 alone, this program facilitated 64 LDLTs, constituting 27.7% of the total 231 LDLTs performed. This paper presents the world's first two 5-way LPEs and the first 6-way LPE.

*********

Another (not entirely unrelated) domain in which size is important, and exchange involves many pairs, involves the exchange of shells among hermit crabs. See these earlier posts (which included this short video):

xx

Saturday, July 21, 2012