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Qual Life Res (2015) 24:147151 DOI 10.1007/s11136-014-0752-2
BRIEF COMMUNICATION
Sample size for pre-tests of questionnaires
Thomas V. Perneger Delphine S. Courvoisier
Patricia M. Hudelson Angle Gayet-Ageron
Accepted: 1 July 2014 / Published online: 10 July 2014 Springer International Publishing Switzerland 2014
AbstractPurpose 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 difculties 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 condence 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 was0.05, 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 was0.20. If no problems were observed in a given sample, the upper limit of a two-sided 90 % condence interval reached0.26 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 (515 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.
Keywords Questionnaires Validity Pre-tests Power
Sample size Cognitive interviewing
Introduction
A qualitative pre-test is a key phase of the development, adaptation, or translation of any questionnaire or psycho-metric instrument [16]. The main purpose of the pre-test is to verify that the target audience understands the questions and proposed response options as intended by the researcher, and is indeed able to answer meaningfully. Typically, when pre-testing a self-report instrument, the pre-test participant will rst ll in the questionnaire, and then debrief about each item in sequence. Many researchers also employ more or less extensive cognitive interviewing methods [1, 6]. Identication of problemse.g., unclear question, unfamiliar word, ambiguous syntax, missing time-frame, lack of...