Drawing a Probability Sample of Female Street Prostitutes in Los Angeles County

Journal of Sex Research, Feb, 1999 by David E. Kanouse, Sandra H. Berry, Naihua Duan, Janet Lever, Sally Carson, Judith F. Perlman, Barbara Levitan

Female prostitutes are known to play an important role in the epidemiology of certain sexually transmitted diseases (STDs), both because of the sexual activity involved in their work and because some female prostitutes inject drugs or have sex with men who inject drugs (Darrow, 1990; Miller, Turner, & Moses, 1990; Plummer & Ngugi, 1990). Nevertheless, little of what is known about the size of this population or their risk behaviors has been derived from careful scientific study. Most studies of prostitutes rely on samples of convenience, typically recruiting in jails, STD clinics, and methadone maintenance programs (Astemborski, Vlahov, Warren, Solomon, & Nelson, 1994; Cohen, Alexander, & Wofsy, 1988; Gellert, Maxwell, Higgins, Pendergast, & Wilker, 1993; Khabbaz et al., 1990). A few studies also include outreach recruitment of respondents in areas known for street prostitution (Cohen et al., 1988; Khabbaz et al., 1990; Tabet, Palmer, Wiese, Voorhees, & Pathak, 1992). The biases in these convenience samples are unknown.

The usual way to minimize sampling bias is through the use of probability sampling techniques. However, the nature of commercial sex work makes that approach especially difficult. Because prostitution is an illicit activity, registries or rosters of prostitutes are not available, and screening of persons in the general population who are willing to admit to such activity is inefficient and unlikely to yield satisfactory coverage of the target population. Recruitment of subpopulations found in convenient locations, such as jails and methadone maintenance clinics, may not be representative of the larger population.

Prostitutes can make themselves available to their clients in various ways, including being present in locations where clients are likely to seek them out or notice them. When these locations are public places, prostitutes are also potentially available for sampling by researchers. One way to conduct probability sampling of prostitutes is to take a two-stage sampling approach, in which the first stage consists of sampling locations and time slots from a spatial-temporal sampling frame, and the second stage involves screening and sampling eligible persons at locations and time slots sampled in the first stage. This general approach has been used with success to draw probability samples of other difficult-to-reach, stigmatized populations, such as homeless persons (Koegel, Burnam, & Morton, 1996; Rossi, Fisher, & Willis, 1986). Applying the approach to sample prostitutes extends the method to an illicit population. One obvious challenge is the possible reluctance of prostitutes to acknowledge engaging in illicit activity, a reluctance that could limit the effectiveness of screening to identify cases. Another is the pronounced spatial mobility of the street population in response to law enforcement activities and fluctuations in client availability.

The Los Angeles Women's Health Risk Study (LAWHRS) is the first study of its kind to attempt to draw a probability sample of the prostitute population in a single major metropolitan area. Our goal was to sample prostitutes in a way that permits empirically based estimation of important population characteristics. Separate sampling frames were constructed for street and offstreet segments of the population; however, probability sampling proved infeasible for the offstreet prostitutes because of the difficulty of defining sampling frames for women who solicited clients via advertisement or referrals and our restricted access to those who solicit clients in private places associated with prostitution.

In contrast, we were successful in defining the sampling frame and in contacting and interviewing street prostitutes because they were accessible and could be recruited while they were soliciting clients in public places. This paper describes the methods that were used to construct the sampling frame and to sample and conduct field interviews with street prostitutes. It also describes the field disposition of the street sample. Preliminary data on the relationship between HIV antibody status, prostitute characteristics, and risk and preventive behaviors can be found in Berry, Kanouse, Duan, and Lillard (1992) and in Kanouse, Berry, Duan, Richwald, and Yano (1992). We have also compared the characteristics of the probability sample of street prostitutes with those in convenience samples recruited in jails, drug treatment programs, and STD clinics (Berry, Duan, & Kanouse, 1996).

METHOD

We constructed a sampling frame by systematically interviewing informants knowledgeable about street prostitution in various parts of Los Angeles County. Our primary sampling unit was the "area-day-shift," where area refers to the place of solicitation, day to the day of the week, and shift to a six-hour time period within the day. We randomly sampled area-day-shifts every week for 36 weeks and sent field teams of interviewers, drivers, and phlebotomists to these locations at the selected times. In each area, the field staff randomly selected women on the street, screened them for study eligibility, conducted interviews, and took blood samples. Field staff also conducted an enumeration of all women who could have been approached for screening in that area-day-shift.