This article discusses difference-in-differences (DID) estimators, which are commonly applied in evaluation research. In particular, the discussion focuses on (a) motivation, definition and interpretation of DID estimators, (b) conditions under which DID estimators are valid, (c) data requirements to compute DID estimators, (d) representative applications of DID estimators in the empirical economics literature, (e) extensions of DID estimators, and (f) a simple indirect test to assess the validity of these estimators.
Abadie, Alberto. Difference-in-Difference Estimators. The New Palgrave Dictionary of Economics Online. Ed. Steven N. Durlauf and Lawrence E. Blume. Palgrave Macmillan, December 2009.