In response to my claim that if a school will only take a 5 for placement/credit, that means that a 5 is approximately equivalent to a C in that school’s course…
Nope, I don’t have data to back that up. My assertion is based on the fact that a school will generally give credit if and only if they believe that the student has sufficient background to be successful in the next course in the sequence. In general that level of mastery is a C at most schools. With a D you’d typically get credit but generally not be able (or at least, well-advised) to use it as a prerequisite for a follow-on course, and might not be able to count the course toward your major. (Generally, of course) if a school will only give credit/placement for a 5, it means they don’t think a 4 shows sufficient mastery to move on. So for that school, a 4 on the AP exam may map to approximately D-level mastery at best on the college’s version of the class, and a 5 would map to a C-ish (or better, of course – some 5s are stronger than others). The scores a school will accept for credit in a particular course are probably the best metric we have of alignment between their version of the course and the AP version. Of course, sometimes it means something different like “the AP exam doesn’t quite cover everything we think is important, so even with a 5 you’ll be missing content, but if you got a 5, you’re probably smart enough to catch up.”