Grouping corrections for sequential tests

When testing sequentially with grouped data, we may or may not adjust the test statistic to allow for the grouping. If the test statistic is not adjusted, the whole power curve will be altered by the grouping, while if the statistic is adjusted, the power curve is altered, but the probability levels or errors of the first and second kind, being distribution‐free, are unchanged. It is shown, even for very coarse grouping of the data, that the power curve is only slightly affected by using the unadjusted statistic for tests on the mean or the variance of a normal distribution. For certain other distributions, however, in particular the negative exponential distribution, this procedure breaks down completely if the group width exceeds a certain value.