A confidence interval is an interval of numbers that contains the expected true value of the population parameter that the interval addresses. A 90 percent confidence interval indicates a 90 percent confidence level that the true value is located within the given parameters.
Information from a sample population is used to compute the endpoints of the interval. A large population consists of a sample size of observations that is greater than 30. For example, a sample of 121 observations with a standard deviation of 20 and an average of 50 has a 90 percent confidence interval for the true population average.