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<p>You’re right. Dividing enrollees by admits is just too crude a way to do it.</p>
<p>You try to take into account things like ability (your students who looked at you as a ‘reach’ will be more likely to enroll than those who see you as a safety, for example) and other factors, like major field. If your applicant and/or admit pools shift even a little bit in the balance of such things (as they do sometimes!), your yield could change. </p>
<p>Schools may also try to figure in some other measures of interest, like overall number of apps received, how quickly deposits are flowing in ahead of the deadline, etc.</p>