This guide will help researchers design and implement impact studies in education so that the findings are more generalizable to the study's target population. Guidance is provided on key steps that researchers can take, including defining the target population, selecting a sample of schools--and replacement schools, when needed--managing school…

As a result of the use of random assignment to treatment, randomized experiments typically have high internal validity. However, units are very rarely randomly selected from a well-defined population of interest into an experiment; this results in low external validity. Under nonrandom sampling, this means that the estimate of the sample average…