چکیده :
When designing clinical trials, researchers often encounter the uncertainty in the treatment effect or
variability assumptions. Hence the sample size calculation at the planning stage of a clinical trial may
also be questionable. Adjustment of the sample size during the mid-course of a clinical trial has become a
popular strategy lately. In this paper we propose a procedure for calculating additional sample size needed
based on conditional power, and adjusting the final-stage critical value to protect the overall type-I error
rate. Compared to other previous procedures, the proposed procedure uses the definition of the conditional
type-I error directly without appealing to an extra special function for it. It has better flexibility in setting
up interim decision rules and the final-stage test is a likelihood ratio test.