To what extent are students assigned to classrooms randomly (with respect to ability) and to what extent does prior achievement affect assignments and teacher matches? Hedvig Horvath points out that we should consider two separate aspects of this question.
- Tracking: Are students of like ability grouped into the same classrooms?
- Teacher matching: Do some teachers persistently receive assignments with mostly high-achieving versus low-achieving students?
Horvarth measures such nonrandom assignment using data from North Carolina elementary schools. She finds:
- 60 percent of schools track.
- Half of schools that track match teachers to the same kind of student year after year.
Horvath also offers a reminder about implications for value-added models. The idea of using value added is that we only judge a teacher based on her students’ starting point at the beginning of the year. But if students with relatively low starting points are also likely to progress more slowly, then value-added estimates will be biased.