Human trafficking has long-lasting implications for the well-being of trafficked people, families, and affected communities. Prevention and intervention efforts, however, have been stymied by a lack of information on the scale and scope of the problem. Because trafficked people are mostly hidden from view, traditional methods of establishing prevalence can be prohibitively expensive in the recruitment, participation, and retention of survey participants. Also, trafficked people are not randomly distributed in the general population. Researchers have therefore begun to apply methods previously used in public health research and other fields on hard-to-reach populations to measure the prevalence of human trafficking. In this topical review, we examine how these prevalence methods used for hard-to-reach populations can be used to measure the prevalence of human trafficking. These methods include network-based approaches, such as respondent-driven sampling and the network scale-up method, and venue-based methods. Respondent-driven sampling is useful, for example, when little information about the trafficked population has been produced and when an adequate sampling frame does not exist. The network scale-up method is unique in that it does not target the hidden population directly. The implications of our work internationally include the need for documenting and validating the various prevalence estimation methods in the United States in a more robust way than was done in existing efforts. In providing this roadmap for estimating the prevalence of human trafficking, our overarching goal is to promote the equitable treatment and overall well-being of the socially disadvantaged populations who disproportionately experience human trafficking.

BACKGROUND: Substance use among youth is a major public health concern. Of note, substance use among youth is increasing in prevalence, and the incidence of substance use at earlier ages is rising. Given the long-term consequences of early substance use, it is important to identify factors that increase youth vulnerability to drug use, as they may be important targets for future interventions.
OBJECTIVE: This study aims to use innovative methods, such as venue-based sampling, to recruit youth who are disconnected from school and use community-based participatory research to gain a better understanding of the prevalence of substance use and important correlates among youth aged between 13 and 21 years in Paterson, New Jersey, a low-income, urban community. The study will use a convergent, mixed methods design involving multiple data collection components and the analysis of a ministrative data source, designed with the strengths of complex intervention frameworks in mind. The overall aims of the study are to identify the prevalence of substance use among youth who are engaged in school and not engaged in school; to understand important antecedents and correlates of substance use; and to use this information to inform social, environmental, and culturally appropriate interventions to address substance use and its correlates among youths in a lower-resourced urban community.
METHODS: This study will use both qualitative and quantitative methods to address important questions. Specifically, semistructured interviews using focus group and interview methodologies will be used to assess youths\' lived experiences and will account for specific details that quantitative methods may not be able to attain. In addition, quantitative methods will be used to examine direct and multilevel associations between neighborhood factors and youth substance use and mental health outcomes.
RESULTS: A previous analysis from a substance use initiative in Paterson, New Jersey found that youth who use substances such as marijuana and alcohol are more likely to have higher rates of depression and anxiety. On the basis of the research questions, this study will examine the association between neighborhood characteristics, substance use, and mental health symptoms among youth in Paterson by using quantitative and qualitative methods and will use these findings to inform the adaptation of a community- and evidence-based substance use prevention intervention for these youths.
CONCLUSIONS: The findings of this study will provide an important contribution to understanding the role of socioecological factors in predicting substance use and mental health outcomes among youth in a lower-resourced, urban community. Furthermore, these findings will serve as evidence for the development of a culturally informed, community-based prevention program to address substance use disparities for youth, including those who are truant in Paterson, New Jersey.
UNASSIGNED: PRR1-10.2196/29427.

Efforts to increase HIV testing, diagnosis and care are critical to curbing HIV epidemics among cisgender men who have sex with men (MSM) and transgender women (TW) in low- and middle-income countries (LMIC). We compared the effectiveness of respondent-driven sampling (RDS) and venue-based sampling (VBS) for identifying previously undiagnosed HIV infection among MSM and TW in Tijuana, Mexico.
Between March 2015 and December 2018, we conducted RDS within the social networks of MSM and TW and VBS at venues frequented by MSM and TW to socialize and meet sexual partners. Those reached by RDS/VBS who reported at least 18 years of age, anal sex with MSM or TW, and no previous HIV diagnosis were eligible for HIV testing.
Of those screened following recruitment via RDS (N = 1232; 98.6% MSM; 1.3% TW), 60.8% (749/1232) were eligible for HIV testing and 97.5% (730/749) were tested for HIV infection, which led to the identification of 36 newly diagnosed HIV infections (4.9%). Of those screened following recruitment via VBS (N = 2560; 95.2% MSM; 4.6% TW), 56.5% (1446/2560) were eligible for HIV testing and 92.8% (1342/1446) were tested for HIV infection, which led to the identification of 82 newly diagnosed HIV infections (6.1%). The proportion of new HIV diagnoses did not differ by recruitment method (ratio = 0.81, 95% confidence interval: 0.55 to 1.18). Compared to those recruited via RDS, those tested following recruitment via VBS were younger, more likely to identify as gay, and more likely to identify as TW. Compared to those recruited via VBS, those newly diagnosed with HIV infection following recruitment via RDS reported higher levels of internalized stigma and were more likely to report injection drug use and a history of deportation from the United States.
Despite RDS and VBS being equally effective for identifying undiagnosed HIV infection, each recruitment method reached different subgroups of MSM and TW in Tijuana. Our findings suggest that there may be benefits to using both RDS and VBS to increase the identification of previously undiagnosed HIV infection and ultimately support HIV care engagement among MSM and TW in Mexico and other similar LMIC.

Adolescent girls and young women (AGYW) remain underserved and at risk for HIV acquisition in Ethiopia. However, there is significant risk heterogeneity among AGYW with limited consensus on optimal strategies of identifying vulnerable AGYW. This study assessed the utility of venue-based sampling approaches to identify AGYW at increased risk for HIV infection. Venue mapping and time-location-sampling (TLS) methods were used to recruit AGYW from three sub-cities of Addis Ababa, February-June 2018. Interviewer-administered surveys captured socio-demographic and behavioral characteristics. Measures of AGYW vulnerability were assessed geographically and described by venue type. A total of 2468 unique venues were identified, of which 802 (32%) were systematically selected for validation and 371 (46%) were eligible including many sites that would traditionally not be included as venues in need of HIV prevention services. Overall, 800 AGYW were enrolled across 81 sampled venues. AGYW reached were largely out-of-school (n = 599, 75%) with high proportions of AGYW reporting transactional sex (n = 101, 12.6%), food insecurity (n = 165, 20.7%) and migration (n = 565, 70.6%). Taken together, these data suggest the utility of TLS methods in reaching vulnerable, out-of-school AGYW in Addis Ababa, Ethiopia.

Little is known about fatherhood in middle adolescence. To better understand their sexual health needs, we describe relationship characteristics, perception of masculinity, and associated sexually transmitted infection (STI) risk behaviors in a community-based sample of urban middle adolescent boys who have fathered a child or been involved with a pregnancy.
We used venue-based sampling to recruit 339 boys (aged 14-17 years) in neighborhoods with high STI prevalence. We administered a brief survey on sexual relationship and pregnancy history, STI risk, juvenile justice involvement, and masculinity.
Fifteen percent had either fathered a child or been involved with a pregnancy. In multivariate analysis, controlling for age and ethnicity, adolescent fathers were more likely to be involved with juvenile justice and engage in STI risk behaviors. These included condom nonuse and partner checking a cell phone. Although of borderline significance, older partners, past STI testing, and drug or alcohol use at last sex improved model fit.
Adolescent fathers have distinct relational and sexual health needs. Their specific needs should be targeted by prevention programs.

Venue-based distribution of marijuana has become the normative model in the United States to obtain marijuana. This study examines one-time purchase behaviors at medical marijuana dispensaries (MMDs) to identify potential venue- and individual-level targets for prevention. We used a two-stage, venue-based sampling approach to randomly select patrons exiting 16 MMDs in Los Angeles, California during the spring of 2013. Patrons (N = 595) reported their discrete purchase behaviors during their most recent visit to the sampled MMD. We used hierarchical linear modeling to examine the amount spent on marijuana products, regressed on characteristics of the sampled dispensaries and their patrons. We used hierarchical generalized linear modeling to examine the likelihood of purchasing specific types of marijuana products and total grams of loose-leaf buds purchased. Patrons spent US$41.73 on average, with a range of $0-$330. We observed significant variation in purchase behaviors across MMDs and associations between venues located within high median income census tracts and a higher total amount spent and lower odds of purchasing only loose-leaf buds. The networked distance between a patron\'s home and the sampled MMD was positively associated with the total amount spent and total quantity of buds purchased. We also found significant relationships between medical conditions reported for use in three models: total amount spent, purchase of pre-rolled joints, and total grams of buds purchased. Policy makers may want to explore regulating the availability of specialty items that may be attractive to naïve users, such as pre-rolled joints or edibles, or high-concentration products that may be sought out by regular, heavy users.

Middle adolescent males are a difficult group to recruit for community sexually transmitted infection (STI) prevention research. We describe a process of community engagement, and venue-based sampling of 14-17-year-old adolescent males, and compare rates of STIs and STI risk behaviors by venue.
Community engagement consisted of (1) informational meetings with organizations; (2) participation in community meetings and events; (3) hiring community members as study personnel; and (4) an adolescent advisory board recruited from the community. Venues were identified and assessed at different times of the day and days of the week using a structured tool. At selected venues, males ages 14-17 years were invited to participate in a brief survey and provide a urine sample and an optional anal swab for DNA-based STI testing.
Venues were assessed (n = 249), and 31 were selected for recruitment, including parks, apartment complexes, community events, entertainment venues, a community school, and community programs for LGBT (gay, lesbian, bisexual, transgender) and adjudicated youth. We enrolled 667 participants, average age 15.7 years. Participants reported high rates of sexual and STI risk behaviors, but had low rates of STIs. These rates differed by venue, with more structured venues recruiting youth reporting fewer STI risk behaviors and less structured venues within the highest STI prevalence zip code recruiting youth reporting more STI risk behaviors.
Venue-based sampling is a feasible mechanism to target recruitment and enrollment adolescent males with high STI risk behaviors in community settings, with risk profiles varying by setting.

Annual HIV testing is recommended for high-risk populations in the United States, to identify HIV infections early and provide timely linkage to treatment. However, heterosexuals at high risk for HIV, due to their residence in urban areas of high poverty and elevated HIV prevalence, test for HIV less frequently than other risk groups, and late diagnosis of HIV is common. Yet the factors impeding HIV testing in this group, which is predominantly African American/Black and Latino/Hispanic, are poorly understood. The present study addresses this gap. Using a systematic community-based sampling method, venue-based sampling (VBS), we estimate rates of lifetime and recent (past year) HIV testing among high-risk heterosexuals (HRH), and explore a set of putative multi-level barriers to and facilitators of recent testing, by gender. Participants were 338 HRH African American/Black and Latino/Hispanic adults recruited using VBS, who completed a computerized structured assessment battery guided by the Theory of Triadic Influence, comprised of reliable/valid measures on socio-demographic characteristics, HIV testing history, and multi-level barriers to HIV testing. Logistic regression analysis was used to identify factors associated with HIV testing within the past year. Most HRH had tested at least once (94%), and more than half had tested within the past year (58%), but only 37% tested annually. In both men and women, the odds of recent testing were similar and associated with structural factors (better access to testing) and sexually transmitted infection (STI) testing and diagnosis. Thus VBS identified serious gaps in rates of annual HIV testing among HRH. Improvements in access to high-quality HIV testing and leveraging of STI testing are needed to increase the proportion of HRH testing annually for HIV. Such improvements could increase early detection of HIV, improve the long-term health of individuals, and reduce HIV transmission by increasing rates of viral suppression.

Venue-based sampling is the identification of, and outreach to, locations visited by the population of interest for the purpose of collecting data. The method is frequently used to reach specific populations, commonly referred to as \"hidden populations.\" Medical marijuana users represent a hidden population of persons who use marijuana for medicinal purposes. We examine whether venue-based procedures introduce selection or non-respondent bias into the study. The venue based sampling procedures employed for the UCLA Medical Marijuana Study used a two-stage, venue-based sampling approach. First, analyses were conducted to assess potential bias within dispensaries that agreed to participate in the surveys. Secondly, analyses were conducted to examine differences among patrons who responded to surveys. Overall, selection bias was generally absent among study results. Results also illuminated the minimal respondent bias observed among the survey respondents. Results suggest that the use of dispensaries to access and survey medical marijuana users is a viable option to gather patient information that adequately represents the greater population of medical marijuana users in Los Angeles. Thus, recommendations and conclusions based on findings from venue-based studies of medical marijuana users at dispensary sites serve to impartially inform meaningful research.

Time-location sampling (TLS), also called time-space sampling or venue-based sampling is a sampling technique widely used in populations at high risk of infectious diseases. The principle is to reach individuals in places and at times where they gather. For example, men who have sex with men meet in gay venues at certain times of the day, and homeless people or drug users come together to take advantage of services provided to them (accommodation, care, meals). The statistical analysis of data coming from TLS surveys has been comprehensively discussed in the literature. Two issues of particular importance are the inclusion or not of sampling weights and how to deal with the frequency of venue attendance (FVA) of individuals during the course of the survey. The objective of this article is to present TLS in the context of sampling theory, to calculate sampling weights and to propose design-based inference taking into account the FVA. The properties of an estimator ignoring the FVA and of the design-based estimator are assessed and contrasted both through a simulation study and using real data from a recent cross-sectional survey conducted in France among drug users. We show that the estimators of prevalence or a total can be strongly biased if the FVA is ignored, while the design-based estimator taking FVA into account is unbiased even when declarative errors occur in the FVA.