Definition

Confidence interval

A range of plausible values for a population parameter, constructed so that the procedure captures the true value a stated share of the time (usually 95%).

A 95% confidence interval around an estimate says: if we repeated this study endlessly, 95% of the intervals built this way would contain the true value. Narrow intervals mean precision; intervals that exclude zero correspond to significance at the matching alpha.

The interval is often more informative than the p-value because it shows magnitude and uncertainty at once: "the treatment improved scores by 4 points, 95% CI [1, 7]" tells a reader far more than "p = .012".