Suche

It is well-known that, with an unrepresentative sample, the estimate of a causal effect may fail to characterize how effects operate in the population of interest. What is less well under- stood is that conventional estimation practices for observational studies may produce the same problem even with a representative sample. Specifically, causal effects estimated via multiple regression differentially weight each unit’s contribution. The “effective sample” that regres- sion uses to generate the causal effect estimate may bear little resemblance to the population of interest.