Conditional estimation following a group sequential clinical trial

Recently, an unbiased estimator Emerson Kittelson 1997 and an essentially unbiased estimator Todd, Whitehead Facey 1996 have been developed to analyze a group sequential clinical trial after it has stopped. However, for these methods, the expectation of the estimator is not close to the parameter value when conditioned on the stopping time. We propose an approach conditioned on the stopping time, which reduces the discrepancy. The result leads to a group of estimators with less discrepancy and sometimes less mean squared error as well. The new estimators are compared by simulation to five existing methods in the case of a two-armed clinical trial with normal response.