title: Sample size for pre-tests of questionnaires.
abstract: OBJECTIVE
  To provide guidance regarding the desirable size of pre-tests of psychometric questionnaires, when the purpose of the pre-test is to detect misunderstandings, ambiguities, or other difficulties participants may encounter with instrument items (called «problems»).
  
  
  METHODS
  We computed (a) the power to detect a problem for various levels of prevalence and various sample sizes, (b) the required sample size to detect problems for various levels of prevalence, and (c) upper confidence limits for problem prevalence in situations where no problems were detected.
  
  
  RESULTS
  As expected, power increased with problem prevalence and with sample size. If problem prevalence was 0.05, a sample of 10 participants had only a power of 40 % to detect the problem, and a sample of 20 achieved a power of 64 %. To achieve a power of 80 %, 32 participants were necessary if the prevalence of the problem was 0.05, 16 participants if prevalence was 0.10, and 8 if prevalence was 0.20. If no problems were observed in a given sample, the upper limit of a two-sided 90 % confidence interval reached 0.26 for a sample size of 10, 0.14 for a sample size of 20, and 0.10 for a sample of 30 participants.
  
  
  CONCLUSIONS
  Small samples (5-15 participants) that are common in pre-tests of questionaires may fail to uncover even common problems. A default sample size of 30 participants is recommended.
authors.affiliation: Division of Clinical Epidemiology, University Hospitals of Geneva, 6 rue Gabrielle-Perret-Gentil, 1211, Geneva, Switzerland, thomas.perneger@hcuge.ch.
authors.forename: Thomas V
authors.initials: TV
authors.lastname: Perneger
authors.forename: Delphine S
authors.initials: DS
authors.lastname: Courvoisier
authors.forename: Patricia M
authors.initials: PM
authors.lastname: Hudelson
authors.forename: Angèle
authors.initials: A
authors.lastname: Gayet-Ageron
mesh_terms: D005260:Female; D006801:Humans; D015995:Prevalence; D011594:Psychometrics / Q000379:methods*; D011788:Quality of Life; D018401:Sample Size; D013223:Statistics as Topic / Q000379:methods*; D011795:Surveys and Questionnaires