  Difference-in-differences analysis with a control group that differs
considerably from a treated group is vulnerable to bias from historical events
that have different effects on the groups. Constructing a more closely matched
control group by matching a subset of the overall control group to the treated
group may result in less bias. We study this phenomenon in simulation studies.
We study the effect of mountaintop removal mining (MRM) on mortality using a
difference-in-differences analysis that makes use of the increase in MRM
following the 1990 Clean Air Act Amendments. For a difference-in-differences
analysis of the effect of MRM on mortality, we constructed a more closely
matched control group and found a 95\% confidence interval that contains
substantial adverse effects along with no effect and small beneficial effects.
