In Simple Words
Imagine evaluating a gardener's skill by measuring plant growth, but subtracting points if the plants are placed in a sunny greenhouse. If the gardener deliberately created or earned that ideal environment, penalizing them for the setting hides their actual gardening capability.
Research question
Including classroom-level baseline mean scores as covariates in value-added models strips away variance that actually represents true teacher effectiveness rather than non-random student composition.
Participants
N = 27,255 randomized students in grades 4–8 across 619 randomization blocks in 6 urban school districts.
Study Design
LIML IV regression decomposing value-added model variance to test whether components subtracted by peer effect controls predict student achievement following random assignment.
Results
The component removed by peer effect controls predicted student achievement after random assignment with a coefficient of 1.150 (p < 0.01, SE = 0.336), proving it contains causal teacher effectiveness. In contrast, the component removed by individual baseline score controls yielded a coefficient of 0.047 (SE = 0.042), showing baseline score controls successfully remove non-teacher sorting noise.
Takeaway
Omit classroom-level baseline mean achievement controls (peer effect covariates) from within-school teacher value-added specifications to prevent subtracting legitimate teacher impacts.
Source and citation
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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 →