StATS: Be sure to account for dropouts in your sample size calculation (December 29, 2006)
I helped out a colleague with an NIH grant, and when the critique came back, it mentioned two issues that I should have been aware of. First, they pointed out the need for an intention-to-treat analysis strategy. Second, they noted the long duration of the study, with a full year of evaluations on any particular patient, and seemed bothered that we presumed that 100% of the patients would complete the full study. This is indeed naive on my part, and the calculation of a revised sample size is quite simple. If you believe that a proportion P less than 1 of the patients will complete the study, take your sample size from the power calculation and divide by P to get a revised sample size that allows for dropouts.
For example, if your sample size calculation indicates that a total of 40 patients are needed, and you expect only 80% of your patients will complete the full study, then plan to enroll 40 / 0.8 = 50 patients.
This page was written by Steve Simon while working at Children's Mercy Hospital. Although I do not hold the copyright for this material, I am reproducing it here as a service, as it is no longer available on the Children's Mercy Hospital website. Need more information? I have a page with general help resources. You can also browse for pages similar to this one at Category: Sample size justification.