Differences scores: regression-based reliable difference and the regression-based confidence interval.

Over 50 years ago Payne and Jones (1957) developed what has been labeled the traditional reliable difference formula that continues to be useful as a significance test for the difference between two test scores. The traditional reliable difference is based on the standard error of measurement (SEM) and has been updated to a confidence interval approach. As an alternative to the traditional reliable difference, this article presents the regression-based reliable difference that is based on the standard error of estimate (SEE) and estimated true scores. This new approach should be attractive to clinicians preferring the idea of scores regressing toward the mean. The new approach is also presented in confidence interval form with an interpretation that can be viewed as a statement of all hypotheses that are tenable and consistent with the observed data and has the backing of several authorities. Two well-known conceptualizations for true score confidence intervals are the traditional and regression-based. Now clinicians favoring the regression-based conceptualization are not restricted to the use of traditional model when testing score differences using confidence intervals.