Abstract
Objective
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).
Results
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.
Conclusion
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.
Keywords
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Article info
Publication history
Published online: December 12, 2011
Accepted:
July 26,
2011
Identification
Copyright
© 2012 Elsevier Inc. Published by Elsevier Inc. All rights reserved.
