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 populationthat may be difficult to reach or involve in research or public health programmes. Applicationof a single term to call these sub-sections of populations implies a homogeneity within distinctgroups, which does not necessarily exist. Different sampling techniques were introduced sofar to recruit hard-to-reach populations. In this article, we have reviewed a range of approachesthat have been used to widen participation in studies.Methods: We performed a Pubmed and Google search for relevant English language articlesusing the keywords and phrases: (hard-to-reach AND population* OR sampl*), (hidden ANDpopulation* OR sample*) and (“hard to reach” AND population* OR sample*) and a consultationof the retrieved articles’ bibliographies to extract empirical evidence from publicationsthat discussed or examined the use of sampling techniques to recruit hidden or hard-to-reachpopulations in health studies.Results: Reviewing the literature has identified a range of techniques to recruit hard-to-reachpopulations, including snowball sampling, respondent-driven sampling (RDS), indigenousfield worker sampling (IFWS), facility-based sampling (FBS), targeted sampling (TS), timelocation(space) sampling (TLS), conventional cluster sampling (CCS) and capture re-capturesampling (CR).Conclusion: The degree of compliance with a study by a certain ‘hard-to-reach’ group dependson the characteristics of that group, recruitment technique used and the subject of interest.Irrespective of potential advantages or limitations of the recruitment techniques reviewed,their successful use depends mainly upon our knowledge about specific characteristics of thetarget populations. Thus in line with attempts to expand the current boundaries of our knowledgeabout recruitment techniques in health studies and their applications in varying situations,we should also focus on possibly all contributing factors which may have an impact onparticipation rate within a defined population group.