Confidence interval: A range of the values, calculated from the sample observations which is believed, with the particular probability, to posses the true parameter value. A 95% confidence interval, for instance, implies that were the estimation process which was repeated again and again, then 95% of the calculated intervals would be expected to include the true parameter value. Note that the stated probability level refers to the properties of the interval and not to the parameter itself which is not considered to be the random variable.