If your university enrolls a lot of undergraduates, April can be the cruelest month. Typically, May 1 is the deadline most institutions use as their candidate’s reply date, when students offered admission must let the college know if they plan to accept the offer to enroll or not.

The percentage of students who accept your offer of admission is called your yield rate. The most selective institutions have the highest yield rates, on average: Harvard, Princeton, Yale, and Stanford, for instance, have rates in the high 70% to low 80% range. The weighted average among private, not-for-profits is about 18%; among the nation’s public land grant universities, it’s about 24%. Yield rate gets a lot of attention; not all of it is good.

I once knew of a university president who brought in one of the large corporate consulting companies with a name you’d most likely recognize, to diagnose how they could do better. They conducted an institutional audit, produced the perfunctory report of substantial length and complexity, and in the end, suggested that the problems with declining enrollment could be solved by doing one thing: Increasing the yield rate.

There is no word about whether they invented the idea of “buy low/sell high” as an investment strategy, but it wouldn’t be beyond the realm of possibility, I suppose. It’s equally brilliant as a strategy, and just as effective.

There are ways to increase yield rates (some are artificial, some are common sense, and some require a lot of analysis), but it’s not a switch you throw, and here’s why:

When an admissions office sends out offers of admission, the analogy is not one of planting saplings in a nursery; it’s one of spreading seeds in a field. With saplings, you control location, spacing, temperature, sunlight, water, and soil quality. Even then, you know not every one you plant will thrive, but your prediction is fairly stable across time. You’re mostly in control.

With seeds, you’re scattering on an open field, in the wild, with no control over water, wind, sunshine, temperature, or even how many birds will eat them. You can make some reasonably good prediction about your success based on where and when you scatter the seeds, but there is a lot beyond your control.

Something as simple as a competitor’s change in financial aid strategy, media focus on your university in the months prior to May 1, or the success of your athletics teams–things mostly beyond your control–can have an effect on your yield. And, as I wrote a while ago, yield is decreasing almost everywhere, based on a very simple concept: Algebra. Students are applying to more colleges than ever before, but in the end, the number who enroll will be mostly fixed, even as applications and admissions surge.

If you want to hire someone to help you figure this out, and they don’t acknowledge the root of the problem, or they suggest that increased retention or decreased “summer melt” is a function of tactical operations, think long and hard before signing that contract.

Of course, you want to do all you can to increase your yield. But looking for that magic switch you throw to make it happen is almost certainly going to end in frustration.