Evidence-appraisal glossary
Fragility Index
The number of patients whose outcomes would have to flip, from non-event to event, to turn a statistically significant trial result into a non-significant one. A small index means the finding rests on just a few events.
Also called: fragility.
A p-value below the threshold tells you a result cleared the significance bar, but not how sturdily it did so. The fragility index recomputes the result after swapping the fewest possible outcomes in the group with fewer events, revealing how close the conclusion sat to the edge. Trials with impressive p-values sometimes have an index of only one or two, meaning a handful of different outcomes would erase the effect. It is a readability aid rather than a formal test, and it applies mainly to two-arm trials with binary outcomes.
This is a plain-language methodology definition for reading research. It is general education, not medical advice.