Prevalence of SARS-CoV-2 Infection among Children and Adults in 15 US Communities, 2021

During January-August 2021, the Community Prevalence of SARS-CoV-2 Study used time/location sampling to recruit a cross-sectional, population-based cohort to estimate SARS-CoV-2 seroprevalence and nasal swab sample PCR positivity across 15 US communities. Survey-weighted estimates of SARS-CoV-2 infection and vaccine willingness among participants at each site were compared within demographic groups by using linear regression models with inverse variance weighting. Among 22,284 persons >2 months of age and older, median prevalence of infection (prior, active, or both) was 12.9% across sites and similar across age groups. Within each site, average prevalence of infection was 3 percentage points higher for Black than White persons and average vaccine willingness was 10 percentage points lower for Black than White persons and 7 percentage points lower for Black persons than for persons in other racial groups. The higher prevalence of SARS-CoV-2 infection among groups with lower vaccine willingness highlights the disparate effect of COVID-19 and its complications.

Improving Underestimation of HIV Prevalence in Surveys Using Time-Location Sampling.

We sought to find a method that improves HIV estimates obtained through time-location sampling (TLS) used to recruit most-at-risk populations (MARPs). The calibration on residuals (CARES) method attributes weights to TLS sampled individuals depending on the percentile to which their logistic regression residues belong. Using a real country database, provided by EMIS-2010, with 9591 men who have sex with men (MSM) and an HIV prevalence of 12.1%, we simulated three populations (termed "pseudo-populations") with different levels of HIV. From each pseudo-population, 1000 TLS samples were drawn, and the HIV prevalence estimated by the TLS method and by the CARES method were recorded and compared with the HIV prevalence of the 9591 men. Results showed that the CARES method improves estimates given by the TLS method by getting closer to the real HIV prevalence.

Factors related to condomless anal intercourse between men who have sex with men: results from a European bio-behavioural survey.

BACKGROUND: Relationship status is an important factor associated with condomless anal intercourse (CAI) amongst men who have sex with men (MSM). METHODS: A multi-centre bio-behavioural survey with MSM was conducted in 13 European cities (n = 4901) exploring factors associated with CAI via bivariate and multivariate multilevel logistic regression analyses. RESULTS: Likelihood of CAI with casual partners was associated with being 'out' to a majority (AOR = 1.19;95% CI 1,1.42); knowing their HIV status (AOR = 1.86; 95% CI 1.25,2.76); using substances (1-2 AOR = 1.39; 95% CI 1.16,1.63, 2+ AOR = 1.81; 95% CI 1.35,2.42); being older (AOR = 0.98; 95% CI 0.97,0.99); successful sero-communication (AOR = 0.79; 95% CI 0.67,0.94); and, not having a recent HIV test (AOR = 0.78; 95% CI 0.66,0.92). CAI with steady partners was associated with successful sero-communication (AOR = 2.72; 95% CI 2.72,3.66); not having a recent HIV test (AOR = 1.26; 95% CI 1.09,1.46), and; being older (AOR = 0.99; 95% CI 0.98,0.99). CONCLUSIONS: Understandings of partner type and/or relationship status in relation to CAI amongst MSM can potentially play an important role in the development of culturally appropriate HIV/STI prevention and risk-reduction efforts targeting at-risk MSM. Our results speak to the need to consider segmented and tailored public health and health promotion initiatives for MSM with differing CAI behaviours and relationship profiles.

Assessing malaria risk at night-time venues in a low-transmission setting: a time-location sampling study in Zambezi, Namibia.

BACKGROUND: Identifying efficient and effective strategies to reach and monitor populations at greatest risk of malaria in low-transmission settings is a key challenge for malaria elimination. In Namibia's Zambezi Region, transmission is ongoing yet its drivers remain poorly understood. A growing literature suggests that night-time social activities may lead to malaria exposure that is beyond the reach of conventional preventive interventions, such as insecticide treated bed nets and indoor residual spraying. METHODS: Formative research was conducted with community members in March, 2015 in the catchment areas of six randomly selected health facilities in the western Zambezi Region to identify night-time locations where large numbers of individuals regularly congregate. Using time-location sampling, a survey was conducted between March and May, 2015 at community-identified venues (bars and evening church services) to develop representative estimates of the prevalence of parasite infection and risk factors among venue-goers. RESULTS: When compared to a contemporaneous household survey of the general population aged 15 and older (N = 1160), venue-goers (N = 480) were more likely to have spent the night away from their home recently (17.3% vs. 8.9%, P = 0.008), report recent fever (65.2% vs. 36.9%, P < 0.001), and were less likely to have sought care for fever (37.9% vs. 52.1%, P = 0.011). Venue-goers had higher, but not significantly different, rates of malaria infection (4.7% vs. 2.8%, P = 0.740). Risk factors for malaria infection among venue-goers could not be determined due to the small number of infections identified, however self-reported fever was positively associated with outdoor livelihood activities (adjusted odds ratio [AOR] = 1.9, 95% CI 1.0-3.3), not wearing protective measures at the time of the survey (AOR = 6.8, 9% CI 1.4-33.6) and having been bothered by mosquitos at the venue (AOR = 2.7, 95% CI 1.5-4). CONCLUSIONS: Prevention measures and continued surveillance at night-time venues may be a useful complement to existing malaria elimination efforts.

Leveraging Geospatial Approaches to Characterize the HIV Prevention and Treatment Needs of Out-of-School Adolescent Girls and Young Women in Ethiopia.

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.

Prevalence of hepatitis C infection, screening and associated factors among men who have sex with men attending gay venues: a cross-sectional survey (PREVAGAY), France, 2015.

BACKGROUND: Over the last 20 years, Hepatitis C virus (HCV) infection prevalence has dramatically increased among HIV-infected men who have sex with men (MSM) in many countries worldwide. It is suspected that this increase is primarily driven by sexual behaviours linked to blood exposure. Monitoring these behaviours is crucial to understand the drivers of the epidemic. This study assessed the prevalence of chronic HCV infection among MSM attending gay venues and associated chronic HCV risk factors. HCV screening and associated factors were described. METHODS: The cross-sectional survey PREVAGAY, based on time-location sampling, was conducted in 2015 among MSM attending gay venues in 5 French metropolitan cities. A self-administered questionnaire was completed and capillary whole blood on dried blood spots (DBS) collected. Possible factors associated with chronic HCV prevalence and with HCV screening in the previous year were investigated using Poisson regression. RESULTS: Chronic HCV infection prevalence from DBS analysis was 0.7% [IC95%: 0.3-1.5] in the study's 2645 participants and was 3.0% [1.5-5.8] in HIV-positive MSM. It was significantly higher in those who reported the following: (lifetime) slamming (with or without the sharing of injection equipment); (during the previous year) fisting and chemsex, unprotected anal intercourse with casual partners, using gay websites and/or of mobile-based GPS applications, and having more than 10 sexual partners. Only 41.3% [38.2-44.5] of the participants reported HCV screening during the previous year. Screening was significantly more frequent in MSM under 30 years of age, those who were HIV-positive, those vaccinated against hepatitis B and meningococcus C, and those who reported the following (during the previous year): more than 10 sexual partners, at least one sexually transmitted infection and fisting. CONCLUSION: Chronic HCV infection prevalence in MSM attending gay venues was significantly higher in HIV-positive MSM and in those with risky sexual behaviours. Reflecting current screening recommendations for specific populations, previous HCV screening was more frequent in HIV-positive individuals and those with risky sexual behaviours. Nevertheless, HCV screening coverage needs to be improved in these populations. Comprehensive medical management, which combines screening and linkage to care with prevention strategies, is essential to control HCV among MSM.

"Starfish Sampling": a Novel, Hybrid Approach to Recruiting Hidden Populations.

We sought to leverage the strengths of time location sampling (TLS) and respondent-driven sampling (RDS) for surveys of hidden populations by combing elements of both methods in a new approach we call "starfish sampling." Starfish sampling entails random selection of venue-day-time units from a mapping of the locations where the population can be found, combined with short chains of peer referrals from their social networks at the venue or presenting to the study site later. Using the population of transmen in San Francisco as a case example, we recruited 122 eligible participants using starfish sampling: 79 at randomly selected venues, 11 on dating applications, and 32 by referral. Starfish sampling produced one of the largest community-recruited samples specifically for transmen to date. Starfish sampling is a flexibility method to recruit and sample hidden populations for whom conventional TLS and RDS may not work in theory or practice.

Time location sampling in men who have sex with men in the HIV context: the importance of taking into account sampling weights and frequency of venue attendance.

Sex between men is the most frequent mode of HIV transmission in industrialised countries. Monitoring risk behaviours among men who have sex with men (MSM) is crucial, especially to understand the drivers of the epidemic. A cross-sectional survey (PREVAGAY), based on time-location sampling, was conducted in 2015 among MSM attending gay venues in 5 metropolitan cities in France. We applied the generalised weight share method (GWSM) to estimate HIV seroprevalence for the first time in this population, taking into account the frequency of venue attendance (FVA). Our objectives were to describe the implementation of the sampling design and to demonstrate the importance of taking into account sampling weights, including FVA by comparing results obtained by GWSM and by other methods which use sample weights not including FVA or no weight. We found a global prevalence of 14.3% (95% CI (12.0-16.9)) using GWSM and an unweighted prevalence of 16.4% (95% CI (14.9-17.8)). Variance in HIV prevalence estimates in each city was lower when we did not take into account either the sampling weights or the FVA. We also highlighted an association of FVA and serological status in the most of investigated cities.

Socio-demographic Characteristics, Sexual and Test-Seeking Behaviours Amongst Men Who have Sex with Both Men and Women: Results from a Bio-behavioural Survey in 13 European Cities.

Within the MSM population, men who have sex with both men and women (MSMW) are identified as a high-risk group both worldwide and in Europe. In a multi-centred bio-behavioural cross-sectional study, we aimed to assess the relationship(s) between socio-demographic factors, stigma, sexual behavioural patterns, test seeking behaviour and sero-status amongst MSMW. A multi-level analysis was conducted to identify factors associated with being MSMW versus Men who have Sex with Men Only (MSMO). A total of 4901 MSM were enrolled across the 13 study sites. Participants were categorised as MSMW in the 12.64% of the cases. Factors such as educational status, perceived homonegativity, testing facilities knowledge and HIV testing lifetime seem to be relevant factors when characterising the MSMW group. The results highlight the vulnerability of MSMW and the wide spectrum of risky behavioural and psycho-social patterns, particularly in terms of HIV testing, 'outness', and perceived stigma.

Design-based inference in time-location sampling.

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.

Vaccine Hesitancy at Nine Community Sites Across the United States, Early in COVID-19 Vaccine Rollout.

BACKGROUND: Vaccine hesitancy has been a significant concern throughout the COVID-19 pandemic. Vaccine hesitancy can be attributed to lack of confidence in vaccines, complacency about the health threat, or lack of convenience of vaccination. To date, few studies have used methods designed to include populations underrepresented in research when identifying factors associated with vaccine hesitancy. METHODS: Between January and July 2021, potential participants were recruited from community venues selected through time-location sampling in 15 defined communities in the United States. Study staff administered a questionnaire on demographics, COVID-19 behaviors and attitudes, and vaccination status or intention to consenting individuals. Vaccine hesitancy was analyzed among those age 18 years and older from nine of the 15 sites and was defined as self-reported neutral, unlikely, or very unlikely vaccine intention. Logistic regression modeling, adjusted for site, identified factors associated with vaccine hesitancy. RESULTS: Among 11,559 individuals, vaccine hesitancy by site ranged from 8.7 to 31.1%. Vaccine hesitancy was associated with being Black compared to White, being White compared to Asian, younger age, unstable housing, being unemployed, lower income, having a disability, providing care in home, not reporting inability to visit sick or elderly relatives during the pandemic, not reporting increased anxiety during the pandemic, and not spending more time with loved ones during the pandemic. CONCLUSIONS: In these selected US communities, early in vaccine rollout, there were significant racial disparities in vaccine hesitancy. Additionally, individuals who were more marginalized due to their socioeconomic status were more likely to report vaccine hesitancy. Vaccine campaigns should make efforts to remove barriers to vaccination, by improving convenience.

Biobehavioral survey using time location sampling among female sex workers living in Ghana in 2020.

Background: The HIV epidemic in Ghana is characterized as a mix of a low-level generalized epidemic with significant contributions from transmission among female sex workers (FSW) and their clients. This study seeks to identify and describe key characteristics and sexual behaviors of FSW and estimate the prevalence of HIV, syphilis, gonorrhea, chlamydia, and hepatitis B virus (HBV) among FSW in Ghana. Method: A total of 7,000 FSW were recruited for the study using Time Location Sampling (TLS) approach with 5,990 (85.6%) participants completing both biological and the behavioral aspects of the study. A structured questionnaire was administered to respondents to assess several factors, such as background characteristics, sexual risk behaviors, condom usage, HIV/AIDS knowledge, opinions, and attitudes. Trained staff conducted face-to-face interviews using mobile data collection software (REDCap) after provision of specimens for HIV and STI testing. Descriptive statistics such as medians, ranges, charts, and percentages are performed and presented. Also included, are bivariate analyses to establish relationships between FSW type and other relevant characteristics of the study. Results: Among the 7,000 (100%) FSW sampled from all regions, 6,773 took part in the behavioral and 6,217 the biological. There were 783 (11.2%) respondents who took part only in the behavioral and 227 (3.2%) only in the biological. Most were young, with a median age of 26 years, majority had never been married or were widowed/divorced and a quarter had no education or had only primary education. Majority (74.8%) of FSW first sold sex at age 25 years or less with a median age of 20 years. Most (84.8%) of the FSW indicated that they entered sex work for money, either for self or family and had an average of eleven (11) sexual partners per week. More than half (55.2%) of the FSW were new entrants who had been in sex work for less than 5 years before the study. Consistent condom use with paying clients was generally unsatisfactory (71%), and was however, very low (24%) with their intimate partners or boyfriends. Only about half (54.6%) of FSW have been exposed to HIV prevention services in the last three months preceding the survey, and this varies across regions. Overall, comprehensive knowledge about HIV and AIDS was low. Only 35% of FSW had comprehensive knowledge. HIV prevalence was 4.6% and was higher among seaters (brothel-based) and older FSW who had been sex work for a longer period. The HIV prevalence from the previous bio-behavioral survey (BBS) in 2015 and 2011 were estimated to be 6.9 and 11.1%, respectively. Conclusion: Compared to the results from the previous studies, the findings give an indication that Ghana is making significant progress in reducing the burden of HIV among FSW in the country. However, risky behaviors such as low consistent condom use, low coverage of HIV services across the regions, and low comprehensive knowledge could reverse the gains made so far. Immediate actions should be taken to expand coverage of HIV services to all locations. Efforts must be made to reach out to the new entrants while also addressing strongly held myths and misconceptions about HIV.

ADAPTIVE TIME LOCATION SAMPLING FOR COMPASS, A SARS-COV-2 PREVALENCE STUDY IN FIFTEEN DIVERSE COMMUNITIES IN THE UNITED STATES.

The COVPN 5002 (COMPASS) study aimed to estimate the prevalence of SARS-CoV-2 (active SARS-CoV-2 or prior SARS-CoV-2 infection) in children and adults attending public venues in 15 socio-demographically diverse communities in the United States. To protect against potential challenges in implementing traditional sampling strategies, time-location sampling (TLS) using complex sampling involving stratification, clustering of units, and unequal probabilities of selection was used to recruit individuals from neighborhoods in selected communities. The innovative design adapted to constraints such as closure of venues; changing infection hotspots; and uncertain policies. Recruitment of children and the elderly raised additional challenges in sample selection and implementation. To address these challenges, the TLS design adaptively updated both the sampling frame and the selection probabilities over time using information acquired from prior weeks. Although the study itself was specific to COVID-19, the strategies presented in this paper could serve as a case study that can be adapted for performing rigorous population-level inferences in similar settings and could help inform rapid and effective responses to future global public health challenges.

Being in the Closet. Correlates of Outness Among MSM in 13 European Cities.

Data for MSM continue to show a high risk of acquiring HIV-STIs. Within this population, outness seems to have an impact on both risk-taking and on health seeking behaviors. The objective of this study was to assess the relationship between socio-demographic, behavioral characteristics, testing behaviors, and outness level among MSM using data from a multi-center bio-behavioral cross-sectional study carried out in 13 EU cities. A multilevel analysis was conducted to identify factors associated with being open ("out") versus not being open ("in"). A total of 4,901 MSM were enrolled in the study and were classified as "out" in 71% of the cases. MSM "out" were more likely to report HIV testing and being reached by HIV prevention programs compared to MSM who were "in." The results confirm the key role of outness in relation to different healthy and risky behavior, ranging from testing to party-drug use.

Injecting drug use during sex (known as "slamming") among men who have sex with men: Results from a time-location sampling survey conducted in five cities, France.

BACKGROUND: In the last decade, European cities saw the development of "slamming," a practice related to chemsex that combines three elements: a sexual context, psychostimulant drug use, and injection practices. Epidemiological data on this practice is still sparse and media attention might have unintentionally distorted the size of this phenomenon. Therefore, we aimed to estimate the prevalence of men practicing slam and to identify factors associated with this practice. METHODS: We used data from the Prevagay 2015 bio-behavioral survey to estimate the prevalence of slamming practices. A time-location sampling was performed among gay-labeled venues in five French cites. Behavioral information was recorded using a self-administered questionnaire. The HIV and HCV serostatus were investigated using ELISA tests on dried blood spots. The factors associated with slamming were assessed using a multiple logistic regression. We applied a weighting mechanism to enhance the generalizability of the estimates. RESULTS: Among the 2646 men who have sex with men (MSM) included in our study, 3.1% reported slamming at least once during their lifetime (95% confidence interval (CI) = 2.2-4.3) and 1.6% (95% CI = 1-2.3) said they participated in a slamming session in the last 12 months. In the multivariate analysis, both HCV and HIV biological status were strongly associated with practicing "slam" in the last 12 months (OR = 13.37 (95% CI = 3.26-54.81) and 4.73 (95% CI = 1.58-14.44), respectively). Furthermore, a ten-point decrease in mental health scores was linked with the practice with an OR of 1.37 (95% CI = 1.08-1.73), indicating poorer mental health. CONCLUSION: Even though slamming seems to involve a relatively small proportion of MSM, the vulnerability of this sub-group is high enough to justify setting up harm reduction measures and specific care. Training health professionals and creating services combining sexual health and drug dependence could be an effective response.

Prevalence of drug use during sex amongst MSM in Europe: Results from a multi-site bio-behavioural survey.

BACKGROUND: Substance use has been consistently reported to be more prevalent amongst Men who have Sex with Men (MSM) compared to the general population. Substance use, in particular polydrug use, has been found to be influenced by social and contextual factors and to increase the risk of unprotected intercourse among MSM. The objective of this analysis was to investigate the prevalence and predictors of drug use during a sexual encounter and to identify specific prevention needs. METHODS/DESIGN: A multi-site bio-behavioural cross-sectional survey was implemented in 13 European cities, targeting MSM and using Time-Location Sampling and Respondent-Driven Sampling methods Multivariable multi-level logistic random-intercept model (random effect of study site) was estimated to identify factors associated with the use of alcohol, cannabis, party drugs, sexual performance enhancement drugs and chemsex drugs. RESULTS: Overall, 1261 (30.0%) participants reported drug use, and 436 of 3706 (11.8%) reported the use of two or more drugs during their last sexual encounter. By drug class, 966 (23.0%) reported using sexual performance enhancement drugs, 353 (8.4%) - party drugs, and 142 (3.4%) the use of chemsex drugs. Respondents who reported drug use were more frequently diagnosed with HIV (10.5% vs. 3.9%) before and with other STIs during the 12 months prior to the study (16.7% vs. 9.2%). The use of all the analysed substances was significantly associated with sexual encounter with more than one partner. DISCUSSION: Substance and polydrug use during sexual encounters occurred amongst sampled MSM across Europe although varying greatly between study sites. Different local social norms within MSM communities may be important contextual drivers of drug use, highlighting the need for innovative and multi-faceted prevention measures to reduce HIV/STI risk in the context of drug use.

Comparing Samples of Men Who Have Sex with Men Recruited Online and in Venues, Jiangsu Province, China, 2013.

PURPOSE: Two common methods to sample men who have sex with men (MSM) for HIV research are venue- and internet-based approaches. However, it is unclear which is best to sample Chinese MSM. METHODS: We conducted side-by-side comparisons of time-location sampling (TLS) and an online sample of MSM in Nanjing, China. RESULTS: TLS-recruited MSM tended to be older and of lower socio-economic status compared to online-recruited MSM, whereas online-recruited MSM reported higher risk behavior and lower frequency of HIV testing. CONCLUSION: Significant differences were observed between the two separate samples. Without a gold standard, the choice of sampling method or recruitment approach should be guided by the segment of the population targeted to be reached.

Approaches to Recruiting 'Hard-To-Reach' Populations into Re-search: A Review of the Literature.

BACKGROUND: 'Hard-to-reach' is a term used to describe those sub-groups of the population that may be difficult to reach or involve in research or public health programmes. Application of a single term to call these sub-sections of populations implies a homogeneity within distinct groups, which does not necessarily exist. Different sampling techniques were introduced so far to recruit hard-to-reach populations. In this article, we have reviewed a range of ap-proaches that have been used to widen participation in studies. METHODS: We performed a Pubmed and Google search for relevant English language articles using the keywords and phrases: (hard-to-reach AND population* OR sampl*), (hidden AND population* OR sample*) and ("hard to reach" AND population* OR sample*) and a consul-tation of the retrieved articles' bibliographies to extract empirical evidence from publications that discussed or examined the use of sampling techniques to recruit hidden or hard-to-reach populations in health studies. RESULTS: Reviewing the literature has identified a range of techniques to recruit hard-to-reach populations, including snowball sampling, respondent-driven sampling (RDS), indigenous field worker sampling (IFWS), facility-based sampling (FBS), targeted sampling (TS), time-location (space) sampling (TLS), conventional cluster sampling (CCS) and capture re-capture sampling (CR). CONCLUSION: The degree of compliance with a study by a certain 'hard-to-reach' group de-pends on the characteristics of that group, recruitment technique used and the subject of inter-est. Irrespective of potential advantages or limitations of the recruitment techniques reviewed, their successful use depends mainly upon our knowledge about specific characteristics of the target populations. Thus in line with attempts to expand the current boundaries of our know-ledge about recruitment techniques in health studies and their applications in varying situa-tions, we should also focus on possibly all contributing factors which may have an impact on participation rate within a defined population group.

Un estudio comparativo de tres métodos de muestreo para poblaciones ocultas ode difícil acceso / A comparison of three methods for sampling hard - to - reach or hidden populations

Los proyectos de investigación que necesitan reclutar participantes pertenecientesa poblaciones “ocultas” o “difíciles de encontrar” requieren de métodos de muestreoque no solo faciliten la recolección de los datos y la confidencialidad, sino tambiénque incrementen la validez externa y permitan hacer inferencias estadísticasapropiadas. Este artículo presenta tres métodos que han demostrado ser útiles enlos trabajos de investigación con estas poblaciones estigmatizadas, clandestinaso de difícil acceso: 1) Muestreo basado en sitios y horarios específicos (VenueBased Time- Location Sampling). 2) Muestreo de áreas en las que se localiza lapoblación blanco o población objeto de estudio (Targeted Sampling). 3) Muestreodirigido por el participante (Respondent Driven Sampling). Luego de analizar lascaracterísticas, ventajas y limitaciones de cada método de muestreo, se presentauna comparación de todos ellos en términos de validez externa, posibilidad deobtener muestras probabilísticas y empleo de investigación etnográfica.

Research projects that need to recruit “hard-to-reach” or “hidden populations”require sampling methods that not only facilitate data collection and confidentialitybut also increase external validity and allow for statistically appropriate inferences.This paper presents three sampling methods useful for researchers who workwith stigmatized hard-to-reach or clandestine populations: 1) Venue Based Time/Location Sampling, 2) Targeted Sampling; and 3) Respondent Driven Sampling.The characteristics, advantages and limitations for each method are analyzed.They are also compared in terms of the possibility of obtaining probabilisticsamples, external validity, and use of ethnographic research.

Projetos de pesquisa que necessitam recrutar participantes pertencentesa povoações “oculta” ou “difíceis de encontrar” requerem de métodos deamostragem que não só facilitem a colheita dos dados e a confidencialidade mastambém que aumentem a validade externa e permitam fazer inferências estatísticasapropriadas. Este artigo teórico apresenta três métodos que demonstraram serúteis nos trabalhos de pesquisa com estas povoações estigmatizadas, clandestinasou de difícil acesso: 1) Amostragem baseado em lugares e horários específicos(Venue Based Time- Location Sampling); 2) Amostragem de áreas nas quais selocaliza a população branco ou população objeto de estudo (Targeted Sampling);e 3) Amostragem dirigido pelo participante (Respondent Driven Sampling).Depois de analisar as características, vantagens e limitações de cada método deamostragem, se apresenta uma comparação de todos eles em termos de validadeexterna, possibilidade de obter amostras probabilísticas e emprego de pesquisaetnográfica.