The Fragility Index Is Typically Misinterpreted and of Low Value: Clinical Trials Are Designed to Be Fragile

The Fragility Index (FI) is defined as the number of patients whose outcome would need to change to reverse a statistically significant finding to a nonsignificant finding. The FI is nothing more than a repackaging of statistical significance based on the P value, perpetuating (1) ignoring of results that are “not” statistically significant; (2) treating results that are statistically significant as certain; and (3) distracting from evaluation of clinical significance. A well-designed trial includes a sample size calculation to determine the minimum number of patients required to observe a difference between study groups (if a difference exists). By including this minimum number, clinical trials are designed to be fragile, yet subsequently criticized as such, leading readers to the incorrect conclusion that the studies are flawed. It’s time to move past systematic reviews focused on the FI.