Let’s take two children going to two different schools whose teachers are equally good. One year, a number of other kids, who happen to be doing worse, move from one school to the other. One school’s proficiency numbers would go up, while the other would go down. But for all of the other kids, nothing may have changed.

A parent considering those two schools, however, would suddenly think one was better or worse than the other, and make choices on that basis. If those other kids influence things like classroom discipline, it might matter, but it could well be that both schools remain just as good as they were before. In short, the numbers are misleading.

[Continue reading David Alpert’s post at Greater Greater Washington.]

David Alpert is founder and editor of Greater Greater Washington. The Local Blog Network is a group of bloggers from around the D.C. region who have agreed to make regular contributions to All Opinions Are Local.