Fragility Index

The fragility index is a powerful and intuitive statistical concept. The index tells you how many people in a study would have had to have a different outcome in order for the study to become “not statistically significant” (to have a p value above 0.05). A fragility index of 100 tell you that 99 patients could have slipped from a good to a bad outcome and the trial would have still been statistically significant. A fragility index of 1, on the other hand, tells you that if even a single patient had a different outcome, the trial would have been reported as negative instead of positive. It is a powerful tool, because it gives you a sense of how easily random chance could have changed the results of a trial. You should be especially wary of any trial where the number of patients lost to follow up is greater than the fragility index.

The fragility index is calculated by iteratively converting one patient from a “non-event” to an “event” in the group with the smallest number of events and recalculating the p value. This is repeated until the p value is ≥ 0.05.

There are some free online calculators that allow you to calculate the fragility index for yourself:

Other FOAMed Resources

The fragility index on The Bottom Line

Fragility index on LifeintheFastlane

A related concept, the instability index, is discussed by Josh Farkas on PulmCrit


Ridgeon EE, Young PJ, Bellomo R, Mucchetti M, Lembo R, Landoni G. The Fragility Index in Multicenter Randomized Controlled Critical Care Trials. Critical care medicine. 44(7):1278-84. 2016. PMID: 26963326

Walsh M, Srinathan SK, McAuley DF. The statistical significance of randomized controlled trial results is frequently fragile: a case for a Fragility Index. Journal of clinical epidemiology. 2014; 67(6):622-8. PMID: 24508144