A confidence interval expresses an estimate as a range rather than a single number, acknowledging the uncertainty of using a sample. It’s built as the estimate plus or minus a margin of error.
What the level means
A 95% confidence level means that if you repeated the sampling many times, about 95% of the intervals you’d construct would contain the true population value. It does not mean there’s a 95% chance the true value is in this one specific interval — a common misreading. Higher confidence (99%) gives a wider interval; lower (90%) a narrower one.
What changes the width
The interval narrows with larger samples and less variability in the data, and widens with higher confidence levels. You can be more confident or more precise, but increasing one tends to cost the other unless you collect more data.
Frequently asked questions
What does a 95% confidence interval mean? About 95% of such intervals, over repeated sampling, would capture the true value.
How do I make an interval narrower? Use a larger sample or accept a lower confidence level.
Is the true value definitely in the interval? Not guaranteed — confidence refers to the long-run method, not one interval.