Stratified analyses for the lifetime estimate were conducted by estimating the population proportion of PWID by sex, race/ethnicity (non-Hispanic white, non-Hispanic black, Hispanic, and other), age group (18–24 years, 25–34 years, 35–49 years, 50–64 years), and for males and females separately by race/ethnic group and by age group. The age groups were determined by the structure of the public use NSDUH data; we restricted all datasets to include only persons aged 18–64 years for comparability. Stratified estimates were for the ever recall period only as there were too few subjects in many of the cells of the cross-classification to produce stable past-year estimates.
