I n a randomized controlled trial (RCT) comparing treatments A and B, a null hypothesis (H 0 ) of no difference in a primary outcome of interest is defined. Whether any observed difference is statistically significant has traditionally been based on the P value and the confidence interval (CI). For a century, an arbitrary threshold of 0.05 (1/20) has been used to define statistical significance. 1 Because this probability is quite low, we conclude that P ≤ 0.05 suggests that the observed difference between A and B is incompatible with H 0 , and, with a ≥ 95% degree of certainty, that A and B are different. The 95% CI is a lower-upper limit in which the true effect estimate lies within a 95% certainty. A 95% CI that does not include the null effect size indicates the observed difference has reached statistical significance, and H 0 is rejected. The 95% CI provides information about the precision of the estimate and complements the P value. Reporting both is recommended.Use of a P value of 0.05 to dichotomize whether treatments A and B are "truly" different is appealing because of its simplicity. Indeed, for all its many limitations, 2 its use actually increased from 1990 to 2015 in MEDLINE and PubMed Central abstracts and articles. 3 Statistical significance carries considerable weight. Researchers, editors and reviewers, readers, and the press tend to become more excited about positive results. 4 However, a shocking number of scientific studies and meta-analyses are not reproducible or replicable. [5][6][7] A reasonable question to ask is: if a positive study was to be repeated, how "easy" might it be for the results to change from being statistically significant to non-significant (and thus, rightly or wrongly, lose some of their appeal)? What if there was, by sheer chance, one or several more (or less) outcome event(s) in one of the comparative groups? Would the coveted statistical significance be lost? In recent years, there has been much interest in a metric-the fragility index (FI), [8][9][10][11] first proposed 3 decades ago 12 -to test the robustness/fragility of statistically significant results. The various applications and implications of the FI are discussed herein.