Confidence interval

A confidence interval is a range of values that likely contains the true result. It shows how precise a study's estimate is.

Definition from Shrinktionary, reviewed by Shariq Refai, MD, MBA, FAPA (last reviewed 2026-06-26). Shrinktionary is the network's language layer and carries the canonical short definition; this page adds the research-context detail below. Full definition on Shrinktionary →

A study reports one number, but that number is an estimate from a sample, not the exact truth. The confidence interval, usually a 95 percent interval, is the range the true value most likely falls within. A narrow interval means a precise estimate. A wide interval means an uncertain one.

The confidence interval is one of the most useful things to look at in any anxiety study, and one of the most ignored. Two trials can report the same effect size, one with a tight interval and one with a wide interval. The first is far more trustworthy.

A trial reports that a treatment lowered anxiety scores by 4 points, with a 95 percent confidence interval of 2 to 6. The best estimate is 4, but the true effect is probably somewhere between 2 and 6.

If a confidence interval for a difference includes zero, the study can't rule out no effect. If an interval for a ratio includes 1, the same is true. A wide interval is a sign of a small or noisy study. Read the interval, not only the headline number.