title: Approaches to Recruiting 'Hard-To-Reach' Populations into Re-search: A Review of the Literature.
abstract: 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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Medical Education Research Centre, R & D Campus, Tabriz University of Medical Sciences, Tabriz, Iran ; Department of Health Education & Promotion, School of Health & Nutrition, Tabriz University of Medical Sciences, Tabriz, Iran.
authors.forename: Abdolreza
authors.initials: A
authors.lastname: Shaghaghi
authors.affiliation: Bruce and John Usher Professor of Public Health, Edinburgh Ethnicity and Health Research Group, Centre for Population Health Sciences, University of Edinburgh, UK.
authors.forename: Raj S
authors.initials: RS
authors.lastname: Bhopal
authors.affiliation: Department of Primary Care Research & Development, University of Edinburgh, UK.
authors.forename: Aziz
authors.initials: A
authors.lastname: Sheikh
keywords: Hard-to-reach populations; capture-recapture; hidden populations; respondent driven sampling; time-location sampling; time-space sampling