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The statistical significance of randomized controlled trial results is frequently fragile: a case for a Fragility Index

Michael Walsh, Sadeesh K Srinathan, Daniel F McAuley, Marko Mrkobrada, Oren Levine, Christine Ribic, Amber O Molnar, Neil D Dattani, Andrew Burke, Gordon Guyatt, Lehana Thabane, Stephen D Walter, Janice Pogue, P J Devereaux
Journal of Clinical Epidemiology 2014, 67 (6): 622-8
24508144

OBJECTIVES: A P-value <0.05 is one metric used to evaluate the results of a randomized controlled trial (RCT). We wondered how often statistically significant results in RCTs may be lost with small changes in the numbers of outcomes.

STUDY DESIGN AND SETTING: A review of RCTs in high-impact medical journals that reported a statistically significant result for at least one dichotomous or time-to-event outcome in the abstract. In the group with the smallest number of events, we changed the status of patients without an event to an event until the P-value exceeded 0.05. We labeled this number the Fragility Index; smaller numbers indicated a more fragile result.

RESULTS: The 399 eligible trials had a median sample size of 682 patients (range: 15-112,604) and a median of 112 events (range: 8-5,142); 53% reported a P-value <0.01. The median Fragility Index was 8 (range: 0-109); 25% had a Fragility Index of 3 or less. In 53% of trials, the Fragility Index was less than the number of patients lost to follow-up.

CONCLUSION: The statistically significant results of many RCTs hinge on small numbers of events. The Fragility Index complements the P-value and helps identify less robust results.

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