Confidence interval
MCAT glossary · Critical Analysis & Reasoning Skills
Definition
A range computed from sample data that, under repeated sampling, would contain the true population parameter the stated proportion of the time (e.g., a 95% CI is constructed by a procedure that, applied to many independent samples, produces an interval containing the parameter in 95% of cases). For any single observed interval, the parameter either is or isn't in it — the 95% refers to the procedure's long-run reliability, not to a probability about the specific interval.
Confidence interval in plain English
A confidence interval is an honest way of saying "we're not sure of the exact number, but it's very likely somewhere in this range." When you measure a whole population from just a small sample — say, guessing a country's average height from 500 people — your estimate won't be perfect, so instead of one number you report a band. The tricky part: a "95% confidence" doesn't mean there's a 95% chance the true value sits in your one specific band. It means the method itself is reliable — if you repeated the whole sampling process over and over, about 95% of the bands you'd build would capture the true value, so it's a good bet that yours did too.
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