Answer: Every statistical value based on a sample has some uncertainty. The confidence interval is one way of conveying our uncertainty about a parameter such as for instance a mean. We also have to say something about how far such an estimator is likely to be from the true parameter value. With a confidence interval, we report a range of numbers, in which we hope the true parameter will lie. The interval is centered at the estimated value, and the width (“margin of error”) is an appropriate multiple of the standard error.