  Difference-in-differences estimation is a widely used method of program
evaluation. When treatment is implemented in different places at different
times, researchers often use two-way fixed effects to control for
location-specific and period-specific shocks. Such estimates can be severely
biased when treatment effects change over time within treated units. I review
the sources of this bias and propose several simple diagnostics for assessing
its likely severity. I illustrate these tools through a case study of free
primary education in Sub-Saharan Africa.
