Confidence interval

It is impossible to study every single person in a given population, so researchers select a sample or sub-group of the population. This means that the researcher can only estimate the parameters (i.e. characteristics) of a population, the estimated range being calculated from a set of sample data. Therefore, a confidence interval is a way to measure how well your sample represents the population you are studying. The probability that the confidence interval includes the true value within a population is called the confidence level of the confidence interval. You can calculate a confidence interval for any confidence level you like, but the most commonly used value is 95%. A 95% confidence interval tells us that we can be 95% certain that the true rate lies somewhere between the lower and upper limits of the confidence interval. (Adapted from