The fragility index is a measure of the robustness (or fragility) of the results of a clinical trial. The fragility index is a number indicating how many patients would be required to convert a trial from being statistically significant to not significant (p ≥ 0.05). The larger the fragility index the better (more robust) a trial’s data are. The intent of the fragility index is to be used in conjunction with the P value, 95% confidence interval, and various measures describing benefit or risk (relative risk reduction, absolute risk reduction, etc).

How Is the Fragility Index Calculated?
The fragility index is calculated by converting one patient in the group (control or experimental group) from a “non-event” to an “event” outcome and recalculating a two-sided Fisher’s exact test until the P value meets or exceeds 0.05. In essence, the calculation describes how many patients would have had to have a different outcome (within the group with the fewest number of events) to make a study’s results not statistically significant.

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