n=1 but at my state school the yield was extremely low and I ended up getting accepted off the mid priority list after about 150 people had dropped their As or their WL statuses. It's a pretty decent school too, not super prestigious but quite well known and really cheap in state. Meanwhile at my out of state private school WL there has been literally zero movement. Both schools overenroll too so I don't get why its so different.
Your n=1 is definitely the exception, but my observation is not universal. In particular, the FL state schools do not have spectacular IS yields, and I can't figure out why, given how inexpensive they are IS. Maybe they don't cut as much slack to their IS applicants, and then those that they accept have better options???
In particular, the IS school you are referring to plans for significant WL movement every year, so it is definitely an exception to the rule. It is a great, relatively inexpensive school, but, it is newish and lower ranked, and definitely has a subpar yield given how relatively good it is. The adcom chooses not to compensate for this by over accepting even more, and plans extensive WL movement instead because it is safer for them in case yields unexpectedly jump, which they apparently think is bound to happen at some point in the future.
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What this school does every year isn't representative, and you can't really draw any conclusions from it, other than, yes, they have gone deeper into their WL earlier this year than in past cycles, which is a definite sign WL movement might be up across the board this year. If they did what most other schools do, however, everyone receiving calls from their upper third would have received initial As months ago. They go through their upper third every year, so, why not just accept them initially? That really is what most other schools do, rather than plan to call around 100 +/- off a WL every year. Congratulations, but you were always going to be accepted. You just didn't find out until this week!
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Nevertheless, what I posted above is my observation across several different states, and several different tiers of schools. Private schools are hit or miss year to year, based on whether yields perform as expected and based on how conservative adcoms are with initial As. The explanation at your private school could be as simple as them not being as conservative as many other schools with initial As this year, and might not have any predictive value as to global WL movement this cycle. JMHO.