Original Article| Volume 65, ISSUE 3, P301-308, March 2012

The size of a pilot study for a clinical trial should be calculated in relation to considerations of precision and efficiency

  • Julius Sim
    Corresponding author. Arthritis Research UK Primary Care Centre, Keele University, Keele, Staffordshire ST5 5BG, UK. Tel.: +44-1782-734253; fax: +44-1782-734255.
    Arthritis Research UK Primary Care Centre, Keele University, Keele, Staffordshire ST5 5BG, UK
    Search for articles by this author
  • Martyn Lewis
    Arthritis Research UK Primary Care Centre, Keele University, Keele, Staffordshire ST5 5BG, UK
    Search for articles by this author
Published:December 12, 2011DOI:



      To investigate methods to determine the size of a pilot study to inform a power calculation for a randomized controlled trial (RCT) using an interval/ratio outcome measure.

      Study Design

      Calculations based on confidence intervals (CIs) for the sample standard deviation (SD).


      Based on CIs for the sample SD, methods are demonstrated whereby (1) the observed SD can be adjusted to secure the desired level of statistical power in the main study with a specified level of confidence; (2) the sample for the main study, if calculated using the observed SD, can be adjusted, again to obtain the desired level of statistical power in the main study; (3) the power of the main study can be calculated for the situation in which the SD in the pilot study proves to be an underestimate of the true SD; and (4) an “efficient” pilot size can be determined to minimize the combined size of the pilot and main RCT.


      Trialists should calculate the appropriate size of a pilot study, just as they should the size of the main RCT, taking into account the twin needs to demonstrate efficiency in terms of recruitment and to produce precise estimates of treatment effect.


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