Crossing the null does not mean “no effect”: a survey of internal medicine physicians on the interpretation of effect estimates with wide confidence intervals

When research studies report treatment effects, they commonly include a CI, which reflects the uncertainty associated with the result. Many clinicians assume that if a CI includes "no effect," the treatment must not work. This is misguided. We surveyed internal medicine residents, fellows, and faculty physicians to see how they interpret treatment effects when CIs are wide or cross commonly used decision thresholds. We found that most clinicians incorrectly concluded there was "no effect" whenever the CI crossed the null, even when the best estimate of the effect suggested benefit or harm. Fewer than one in five clinicians correctly identified an effect in these situations. These findings show that clinicians frequently rely on statistical significance rather than the size and direction of the estimated effect when interpreting study results. This misunderstanding may lead to poor clinical decisions, inaccurate guideline recommendations, and misleading communication with patients. Education and clearer reporting are needed to help clinicians interpret CIs correctly and focus on what the results actually mean for decision-making.