This article reports that with a weak economy and a record number of applications at many campuses, admissions deans have deliberately undershot their targets and lengthened their waiting lists. For months a four-digit number has hovered over Douglas L. Christiansen. It's there when he falls asleep and there when he wakes up. The number is 1,550, the size of the freshman class he must enroll this fall--no more, no less. Although Mr. Christiansen, associate provost for enrollment and dean of admissions at Vanderbilt University, has many qualified applicants, he cannot know exactly how many admitted students will enroll. This year Vanderbilt received a record 16,800 applications, 4,000 more than last year. That spike complicated Mr. Christiansen's enrollment projections, as did Vanderbilt's new residential college, built to house all 1,550 incoming freshmen. So this spring, Mr. Christiansen made a strategic decision. First, Vanderbilt would admit fewer applicants than it needed, guarding against an unexpectedly high number of commitments. Then the university would make up the difference by admitting more students from its waiting list.