Confidence Interval

A confidence interval (CI) is an estimated range of values expected in the population but calculated from a sample. One common use of CIs is in the reporting of standardized test results, such as from the Iowa Test of Basic Skills (ITBS) (Hoover & colleagues, 1993). A student who takes the ITBS receives an overall score on the mathematics subtest. Because the observed test score is an imperfect reflection of the test-taker's true mathematical ability, a test-taker's true (or error-free) score will differ by some amount from the observed test score. As such, a value is added or subtracted from the test score to form a plausible range on which a person's true test score may lie. The interval width indicates the degree of uncertainty ...