There is currently a push to evaluate teacher preparation programs based in part on the value-added of their graduates. Predictably, this is a highly controversial issue, and the research supporting it is, to be charitable, still underdeveloped. At present, the evidence suggests that the differences in effectiveness between teachers trained by different prep programs may not be particularly large (see here, here, and here), though there may be exceptions (see this paper).
In the meantime, there’s an interesting little conflict underlying the debate about measuring preparation programs’ effectiveness, one that’s worth pointing out. For the purposes of this discussion, let’s put aside the very important issue of whether the models are able to account fully for where teaching candidates end up working (i.e., bias in the estimates based on school assignments/preferences), as well as (valid) concerns about judging teachers and preparation programs based solely on testing outcomes. All that aside, any assessment of preparation programs using the test-based effectiveness of their graduates is picking up on two separate factors: How well they prepare their candidates; and who applies to their programs in the first place.
In other words, programs that attract and enroll highly talented candidates might look good even if they don’t do a particularly good job preparing teachers for their eventual assignments. But does that really matter?