Planning the duration of accrual and follow-up for clinical trials.

In clinical trials, the duration of accrual and follow-up are chosen to ensure an adequate power to detect a specified difference at a given level of significance, but appropriate choices for durations of accrual and follow-up are not unique. Methods of determining appropriate combinations for the accrual and follow-up periods are given and the unique cost effective choice of accrual and follow-up periods is presented. The accrual period can be substantially decreased by including a short follow-up period. The unique cost effective duration of follow-up depends on the ratio of the cost of accruing patients to the cost of following patients for end point determination.