Changes

Note that the same county can be in multiple pairs, and therefore is in the sample multiple times. This creates the need for multidimensional clustering as developed by Cameron et al, 2006. Standard errors are clustered for both within-state over time correlations of county residuals, as well as within borer-segment. Discussion of these in first paragraph of Section 5 on page 22.

The author goes on to state that <math>\beta</math> is supposed to represent the effect of an additional poll tax or literacy test within a state. He also notes that the laws are highly collinear -- ie, they tend to co-exist together -- and there is little information in the independent dummies.