  I evaluate the performance of estimators that exploit regional variation in
wage levels to identify the employment and wage effects of national minimum
wage laws. For the "effective minimum wage" design, I show that the
identification assumptions in Lee (1999) are difficult to satisfy in settings
without regional minimum wages. For the "fraction affected" design, I show that
economic factors such as skill-biased technical change or regional convergence
may cause parallel trends violations and should be investigated using
pre-treatment data. I also show that this design is subject to misspecification
biases that are not easily solved with changes in specification.
