for regression analyses that included period as a covariate, we used an alternative approach to estimating the control population. In this second approach, we used estimates of the population for every year between 1990 and 2004 to produce a “time-averaged” population sample. The sample was assembled by combining the 1990 and 2000 U.S. census samples with the 2004 American Community Survey sample. Linear interpolation was used to estimate population data for intercensal years. Further details are provided in the Supplemental Material.
