Considerations in determining sample size for pilot studies

College of Nursing, University of Nebraska Medical Center, Lincoln Division, 1230 O Street, Suite 131, P.O. Box 880220, Lincoln, NE 68588-0220, USA.
Research in Nursing & Health (Impact Factor: 1.27). 04/2008; 31(2):180-91. DOI: 10.1002/nur.20247
Source: PubMed


There is little published guidance concerning how large a pilot study should be. General guidelines, for example using 10% of the sample required for a full study, may be inadequate for aims such as assessment of the adequacy of instrumentation or providing statistical estimates for a larger study. This article illustrates how confidence intervals constructed around a desired or anticipated value can help determine the sample size needed. Samples ranging in size from 10 to 40 per group are evaluated for their adequacy in providing estimates precise enough to meet a variety of possible aims. General sample size guidelines by type of aim are offered.

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    • "Based on Cohen's recommendation [21], a total of 128 participants will be required for a full scale RCT of medium-sized effect with power = 0.8 and p = 0.05. For a feasibility study, using an upper bound of 40%, a total of 52 participants will be required [22]. As the intervention program for this study would be carried out over a period of three months and involve twice-weekly participation, attrition was expected. "
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    • "A convenience sample of 40 individuals with HIV/ AIDS was enrolled. This sample size allowed for assessment of the methodology, acceptability of the intervention, and identifying problems with logistics and data-collection strategies (Hertzog, 2008). The sample size also ensured that at least 10 individuals in each cohort group (n = 20) would complete the feasibility study, even with an attrition rate of 50% (Gucciardi, DeMelo, Offenheim, Grace, & Steward, 2007). "

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