confidence interval

A confidence interval expresses how precisely a study measured what it set out to measure. The Cochrane Handbook describes it as a range of values within which we can be reasonably sure that the true effect actually lies. A narrow interval means the estimate is tightly pinned down. A wide one means the study is compatible with a broad span of possible truths, and that width is information, not decoration.

The usual form is a 95 percent confidence interval, and its meaning is easy to misstate. The NIST handbook of statistical methods is explicit that it does not mean there is a 95 percent probability that this particular interval contains the true value. It means that if the same study were repeated many times, about 95 percent of the intervals produced that way would contain it. The confidence belongs to the method, not to the single interval in front of you.

The interval matters most when it crosses the point of no effect. That is not a demonstration that a treatment does nothing. Cochrane warns directly against confusing no evidence of an effect with evidence of no effect: an interval crossing zero usually means the trials were too small to settle the question either way.