Data from the first two rounds of the Community Tracking Study household survey show that coverage expansions through the State Children's Health Insurance Program (SCHIP) have virtually eliminated differences across communities in children's eligibility for public or private health coverage. Nevertheless, some communities continue to have very high rates of uninsured children, in large part be...

External validity is a challenge in treatment effect estimation. Even in randomized trials, the experimental sample often differs from the population of interest. If participation decisions are explained by observed variables such differences can be overcome by reweighting. However, participation may depend on unobserved variables. Even in such cases, under a common support assumption there exi...