In Simple Words
If one coach receives elite varsity athletes while another coach receives untrained beginners, directly comparing their end-of-season win totals misattributes student talent to coaching quality. Adjusting for incoming skill levels resets the starting line so both coaches are evaluated solely on the improvement generated during the season.
Observed teachers before the trial
- Later trial teachers
- Math starting-score spread: 0.382
- Other teachers in their schools
- Math starting-score spread: 0.520
Pretrial teacher records within schools, in student standard deviations. This does not measure whether randomization removed sorting.
See source →Research question
Non-random assignment of students to classrooms causes systematic baseline achievement differences across instructors, biasing unadjusted outcome measures by conflating student background with instructor quality.
Participants
1,181 randomized MET project teachers, 3,802 non-randomized MET school teachers, and 17,153 non-MET school teachers across 6 urban school districts.
Study Design
Within-school baseline achievement covariance analysis across academic years and comparative IV estimation of value-added models with versus without baseline test controls.
Results
Prior to randomization, the within-school standard deviation of teacher mean baseline math scores was 0.382 to 0.520. Evaluating instructors using unadjusted end-of-year student test scores yielded a severely biased predictive coefficient of 0.228 (SE = 0.042), significantly overstating true instructor impact.
Takeaway
Control for prior baseline student achievement test scores in evaluation models to eliminate selection bias caused by classroom sorting.
Source and citation
See source →Read alongside the research
Plain-English glossary
Original explanations, informed by the references below. Examples are illustrations, not data from the studies above. The research text stays unchanged.
- Baseline
The starting measurements in a study. Researchers use these to compare what participants were like at the start with what happens later.
ExampleBefore a training programme starts, researchers measure workers’ earnings. That first measurement is the baseline.
GET-IT glossary · Baseline characteristics ↗Read more about baseline →