Table 1. Description of eligible studies for meta-analysis of the population proportion of PWID in the United States.

Study Name	Population Surveyed	Sampling Method	Data used in meta-analysis	Data collection method	Recall Period	Question wording to define PWID
General Social Survey (GSS)	Persons aged ≥18 years, who spoke English or Spanish*	Probability sample	2000, 2002, 2004, 2006, and 2008§	(2000) Face-to-face interviews using paper instruments; (2002–2008) CAPI.	Ever	Have you ever, even once, taken any drugs by injection with a needle (like heroin, cocaine, amphetamines, or steroids)? Do not include anything you took under a doctor's orders.
National Health and Nutrition Examination Survey (NHANES)	Persons aged 12 to 69 years, who spoke English or Spanish†	Stratified, multistage probability cluster design	1999–2008	ACASI	Ever, Past year	Ever (1999–2004): Have you ever used a needle to take street drugs? (2005–2008): Have you ever, even once, used a needle to inject a drug not prescribed by a doctor? Past year: (1999–2004) In the past 12 months, how many days have you used a needle to take street drugs? (Days greater than 0 indicated injection drug behavior); (2005–2008): How long ago has it been since you last used a needle to inject a drug not prescribed by a doctor? (Time periods of 12 months or less indicated injection drug behavior)
National Survey of Family Growth (NSFG)	Persons aged 15–44 years who spoke English or Spanish	Multistage area probability sample	2002, 2006–2008¶	ACASI	Past year	2002: During the last 12 months, how often have you taken nonprescription drugs using a needle, that is, you took them only for the experience or feeling it caused? This includes ‘shooting up’ and ‘skin-popping.’ (Frequencies greater than 'never' indicated injection drug behavior) 2006: During the last 12 months, how often have you shot up or injected drugs other than those prescribed for you? By shooting up, we mean anytime you might have used drugs with a needle, by mainlining, skin-popping, or muscling. (Frequencies greater than 'never' indicated injection drug behavior)
National Survey of Drug Use and Health (NSDUH)	Persons aged ≥12 who spoke English or Spanish	Stratified, multistage probability cluster sample	2002–2009	ACASI	Ever, Past year	Ever: Have you ever, even once, used a needle to inject heroin? Past year: How long has it been since you last used a needle to inject heroin? (also asked for a) cocaine, b) methamphetamine, c) other stimulants, and d) other drugs not prescribed/took for the feeling or experience. (1) Within the past 30 days; (2) More than 30 days ago but within past 12 months; (3) More than 12 months ago

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.