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The fragility index is a useful metric for evaluating the robustness of cardiovascular clinical trial results, as many statistically significant findings hinge on a small number of events.
The concept of statistical fragility has gained traction in recent years, particularly in the evaluation of clinical trial data. The term “fragility index” (FI) was coined in 2014, when investigators from McMaster University examined 399 high-impact randomized controlled trials (RCTs) to determine how many outcome events would have to be flipped in order to render a trial result statistically non-significant (Figure 1).1 They found that in more than a quarter of RCTs, a change in three or fewer events (ie, a FI≤3) was all that would be required to nullify the trial, with a median FI of eight events across all trials studied.
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Kosyakovsky et al. (2026) studied this question.
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