title: [Analysis of selection bias in the pilot study of a longitudinal study on aging in Spain].
abstract: OBJECTIVE
  To demonstrate that selection of a probabilistic sample at a national level for a study of aging in Spain is subject to selection bias. To quantify the losses produced after each phase of the administration of a questionnaire.
  
  
  METHODS
  We performed a cross-sectional study of the Spanish community-dwelling population aged 50 years or older between 2010 and 2011. Through multivariate logistic regressions, the characteristics of the census tract of the patients' residence were compared between those who agreed (n = 5,813) or refused (n = 7,023) to be included in the sampling frame and between those who agreed (n = 1,677) or refused (n = 2,875) to participate in the study. The individual characteristics of persons who responded (n = 1,398) or refused to respond (n = 346) to a face-to-face questionnaire administered after a telephone interview were also compared. In addition, the reasons for refusal were studied.
  
  
  RESULTS
  The most frequent specific reasons for refusing to be included in the sampling frame or to participate in the study were poor health and disability (14.4% and 27.9%, respectively). In both cases, refusal was more frequent in the census tracts of districts with a lower socioeconomic level or those located in Catalonia, Guipúzcoa or Biscay. Individuals older than 81 participated less frequently in the face-to-face questionnaire. Between 8.6% and 18.4% of participants were lost at each stage of information retrieval.
  
  
  CONCLUSIONS
  Probabilistic sampling in sampling points chosen by the researchers would allow more resources to be devoted to increasing response rates among the groups who are less likely to participate. Questions should be concentrated in only one shorter questionnaire, administered before blood extraction.
authors.affiliation: Fundación INGEMA-Instituto Gerontológico MATIA, San Sebastián/Donosti-Madrid, España. angel.rodriguez@ingema.es
authors.forename: Ángel
authors.initials: Á
authors.lastname: Rodríguez Laso
authors.forename: Elena
authors.initials: E
authors.lastname: Urdaneta Artola
authors.forename: Miguel
authors.initials: M
authors.lastname: de la Fuente Sánchez
authors.forename: Eva
authors.initials: E
authors.lastname: Galindo Moreno
authors.forename: José Javier
authors.initials: JJ
authors.lastname: Yanguas Lezáun
authors.forename: Vicente
authors.initials: V
authors.lastname: Rodríguez Rodríguez
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D000375:Aging; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D008297:Male; D008875:Middle Aged; D010865:Pilot Projects; D012107:Research Design / Q000706:statistics & numerical data*; D015983:Selection Bias; D013030:Spain; D011795:Surveys and Questionnaires
keywords: Aging; Censos; Censuses; Cross-sectional studies; Envejecimiento; Estudios transversales; Negativa a participar; Refusal to participate; Selection bias; Sesgo de selección