Waiting lists from the colleges' point of view

May 1 was the deadline by which students admitted to colleges were supposed to send in their acceptances, so, later this week, when all the mail is in, colleges will finally be in a position to see how many students they might like to admit from their waiting lists.

Unlike rejection letters, which break off a relationship, letters informing candidates that they are waitlisted are meant to influence future behavior. One important goal of the waiting list letter is to encourage those who are most likely to attend if given a (late) offer to signal their willingness in a way that might reliably identify those who will accept an offer from the waiting list if one is offered. That way, if the college has to hurry to fill its places from its waiting list, it can make offers to those most likely to come.

Rice University's FAQ about their waiting list is interesting both for what it reveals and what it conceals:

"What is an "alternate?" What is the "waiting list?"
Your application received strong enough support from the Admission Committee to be approved for admission. However, Rice's small size (only 850 new students) prevents us from admitting all such students. As an “alternate” to the class you are given the opportunity to be placed on the “waiting list.” Students on the waiting list are considered for positions in the class that remain unfilled in May and June.
How many students are on the waiting list?
No one knows yet. Only students who respond affirmatively by May 1, 2009 will be on the waiting list. Not all of the students offered this opportunity will choose to remain an alternate, so the actual size of the waiting list will not be known until May.
How many offers will be made to wait-listed students?
The number of available spaces cannot be determined until after all admitted students respond to our offer of admission by May 1. If fewer than 850 students accept our offer of admission by that date, we will make offers from the waiting list to fill the class in May or June. Over the last four years, 174 students have been admitted from the wait list.
Is the waiting list rank-ordered?
No. All wait-listed students will be reviewed for all available spaces.
Is it appropriate for me to accept an offer of admission from another school while I wait to hear from Rice? What about other waiting lists?
Yes. All schools expect students to commit to one (and only one) school by May 1. However, students may remain on any school’s waiting list throughout the summer. The only rule is that you must withdraw your admission deposit from a school once you accept another school’s offer – you may commit to only one school at a time.
If admitted from the waiting list, how will I be notified?
Students admitted from the waiting list are contacted by telephone. Be sure we have the home phone number and cell phone number (if available) where you can be reached throughout the months of May and June. If you are planning a trip away from home, send a contact number where you can be reached to riceapps@rice.edu.
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What can I do to increase my chances of admission?
You should update your file in writing via mail or email, informing us of your interest in Rice or spring term accomplishments.
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It is our goal to send an update to all wait-listed students by June 1, 2009. The waitlist process will be concluded by July 1."

Update (5/8/09): A Crimson story has some information on Harvard's waitlist plans: Yield Holds Steady For 2013 :
"The first round of waitlist deliberations will run from today until the 19th. Fitzsimmons said that “at least 85 if not more” will be taken off the list in the coming weeks, as the office is still waiting to hear from some admitted students. The admissions office aims to have all decisions out by July 1, Fitzsimmons said. He added that it is possible that students admitted off the waitlist will begin hearing as early as today, but more should find out by the middle of next week. The number of students admitted from the waitlist will depend in part on the number of students who choose to defer their offers of admission for a year. So far, 31 students have elected to defer, a number that the admissions office expects to rise. This number is usually between 30-50 students and does not seem to be affected by any particular variable, Fitzsimmons said, though he added it was possible that the bad economy could cause fewer people to defer this year. "