Description of eligible studies for meta-analysis of the population proportion of PWID in the United States.

CAPI = Computer Assisted Personal Interview; ACASI = Audio Computer-Assisted Self Interview.*Spanish GSS interviews began in 2006.†The public dataset used in this analysis is limited to persons aged 20–69 years, and only those 20–59 answered drug use questions.§GSS data are collected every two years.¶Between 1973 and 2002, NSFG data were collected in cycles. Since 2006, NSFG data are continuously collected. This analysis included data from Cycle 6 (2002) and 2006 through 2008.