In several posts, I’ve complained about how, in our public discourse, we misinterpret changes in proficiency rates (or actual test scores) as “gains” or “progress," when they actually represent cohort changes—that is, they are performance snapshots for different groups of students who are potentially quite dissimilar.
For example, the most common way testing results are presented in news coverage and press releases is to present year-to-year testing results across entire schools or districts – e.g., the overall proficiency rate across all grades in one year compared with the next. One reason why the two groups of students being compared (the first versus the second year) are different is obvious. In most districts, tests are only administered to students in grades 3-8. As a result, the eighth graders who take the test in Year 1 will not take it in Year 2, as they will have moved on to the ninth grade (unless they are retained). At the same time, a new cohort of third graders will take the test in Year 2 despite not having been tested in Year 1 (because they were in second grade). That’s a large amount of inherent “turnover” between years (this same situation applies when results are averaged for elementary and secondary grades). Variations in cohort performance can generate the illusion of "real" change in performance, positive or negative.
But there’s another big cause of incomparability between years: Student mobility. Students move in and out of districts every year. In urban areas, mobility is particularly high. And, in many places, this mobility includes students who move to charter schools, which are often run as separate school districts.
I think we all know intuitively about these issues, but I’m not sure many people realize just how different the group of tested students across an entire district can be in one year compared with the next. In order to give an idea of this magnitude, we might do a rough calculation for the District of Columbia Public Schools (DCPS).