A confidence interval is a measure of estimation. It is an estimated range of values that is likely to include the population parameter being calculated. For instance, instead of estimating the mean age of a certain population to be a single value like 25.5 years, we could say that the mean age is somewhere between 23 and 28. This confidence interval contains the single value we are estimating; yet it gives us a wider net to be right. Related to the confidence interval is the confidence level, which is a measure of the likelihood that our confidence interval will contain the population parameter. Read more about confidence intervals and confidence levels, including examples of both and how to calculate them.
Other new related posts that you might be interested in: Lambda and Gamma.