Epi Lesson: Adjustment of Confidence Intervals for Interim Analyses
Interim analyses are commonly planned in large studies. This can be useful, for example, to ensure patient safety (either by finding a significant benefit or harm early on), or as a cost saving measure by stopping the study early if there is no statistical probability to find a difference between groups later on. There are caveats to interim analyses. It is possible for a perceived “futile” study to become significant if allowed to continue on (lack of power to find a difference early on). It is also possible to find a significant difference early on by chance alone. Because the likelihood to find a difference by chance alone increases every time you analyze the data, there are strategies to account for this (such as the O’Brien Fleming Approach), most of which include raising the bar for statistical significance (i.e. p-value much smaller than 0.05 or using >95%CI instead).