We document that rotation group bias—the tendency for the unemployment rate to vary systematically by month in sample—in the Current Population Survey (CPS) has worsened over time. Estimated unemployment rates for earlier rotation groups have grown sharply relative to later rotation groups; both should be nationally representative samples. This bias increased discretely after the 1994 CPS redesign, and rising nonresponse rates are likely a significant contributor. Survey nonresponse increased after the redesign, mirroring the evolution of rotation group bias. Consistent with this explanation, rotation group bias for households that responded in all eight interviews remained stable over time.

We assess whether welfare reform affects earnings only through mean impacts that are constant within but vary across subgroups. This is important because researchers interested in treatment effect heterogeneity typically focus on estimating mean impacts that only vary across subgroups. Using a novel approach to simulating treatment group earnings under the constant mean impacts within subgroup model, we find this model does a poor job of capturing treatment effect heterogeneity for Connecticut's Jobs First welfare reform experiment. Notably, ignoring within-group heterogeneitywould lead one to miss evidence that treatment effects are consistent with basic labor supply theory.