Confidence Interval

A confidence interval (CI) is a range of plausible values for an unknown population quantity, built so that the procedure captures the true value a stated fraction of the time — typically 95%. A poll might report 53% support with a 95% CI of 50% to 56%.

The 95% is the slippery part. It describes the long-run reliability of the method, not a single interval: if you repeated the whole study many times, about 95% of the intervals you build would contain the truth. It is a misconception to say “there is a 95% probability the true value is inside this particular interval” — for any given interval the truth is either in it or not.