Assessing the impact of additional follow-up in cohort studies

An approach is described for predicting the statistical value of extending follow-up in a cohort study. A simple approximation to the expected number of new events of interest is given. The effect of these events on inferences for parameters such as a standardized mortality ratio is approached in two ways. The first concerns the probability of reversing the conclusion of a significance test. The second approach finds the plausible range of values for the standardized mortality ratio after further follow-up that are consistent with the currently available data. Each of these values is displayed together with the precision of the estimate. The methods are illustrated with results from the International Radiation Study of Cervical Cancer (IARC Scientific Publication No. 52, 1984).