title: Hard-to-reach populations of men who have sex with men and sex workers: a systematic review on sampling methods.
abstract: BACKGROUND
  In public health, hard-to-reach populations are often recruited by non-probabilistic sampling methods that produce biased results. In order to overcome this, several sampling methods have been improved and developed in the last years. The aim of this systematic review was to identify all current methods used to survey most-at-risk populations of men who have sex with men and sex workers. The review also aimed to assess if there were any relations between the study populations and the sampling methods used to recruit them. Lastly, we wanted to assess if the number of publications originated in middle and low human development (MLHD) countries had been increasing in the last years.
  
  
  METHODS
  A systematic review was conducted using electronic databases and a total of 268 published studies were included in the analysis.
  
  
  RESULTS
  In this review, 11 recruitment methods were identified. Semi-probabilistic methods were used most commonly to survey men who have sex with men, and the use of the Internet was the method that gathered more respondents. We found that female sex workers were more frequently recruited through non-probabilistic methods than men who have sex with men (odds = 2.2; p < 0.05; confidence interval (CI) [1.1-4.2]). In the last 6 years, the number of studies based in middle and low human development countries increased more than the number of studies based in very high and high human development countries (odds = 2.5; p < 0.05; CI [1.3-4.9]).
  
  
  CONCLUSIONS
  This systematic literature review identified 11 methods used to sample men who have sex with men and female sex workers. There is an association between the type of sampling method and the population being studied. The number of studies based in middle and low human development countries has increased in the last 6 years of this study.
authors.affiliation: Lúrio University, Rua 4250, Km 2.3, Marrere, Nampula, Mozambique. anabdebarros@gmail.com.|Global Health and Tropical Medicine, Instituto de Higiene e Medicina Tropical, Universidade Nova de Lisboa, Rua da Junqueira 100, Lisbon, Portugal. anabdebarros@gmail.com.
authors.forename: Ana B
authors.initials: AB
authors.lastname: Barros
authors.affiliation: Global Health and Tropical Medicine, Instituto de Higiene e Medicina Tropical, Universidade Nova de Lisboa, Rua da Junqueira 100, Lisbon, Portugal. sfdias@ihmt.unl.pt.
authors.forename: Sonia F
authors.initials: SF
authors.lastname: Dias
authors.affiliation: Global Health and Tropical Medicine, Instituto de Higiene e Medicina Tropical, Universidade Nova de Lisboa, Rua da Junqueira 100, Lisbon, Portugal. mrfom@ihmt.unl.pt.
authors.forename: Maria Rosario O
authors.initials: MR
authors.lastname: Martins
mesh_terms: D015706:Bibliometrics; D035843:Biomedical Research / Q000706:statistics & numerical data; D003906:Developing Countries / Q000706:statistics & numerical data*; D005260:Female; D018451:Homosexuality, Male / Q000706:statistics & numerical data*; D006801:Humans; D020407:Internet; D008297:Male; D015233:Models, Statistical; D018579:Patient Selection; D010506:Periodicals as Topic / Q000706:statistics & numerical data*; D015983:Selection Bias; D060386:Sex Workers / Q000706:statistics & numerical data*