Reliability of Difference Scores

A standalone Windows program that calculates the reliability of difference scores, given the reliability of two tests and the correlation Between those two tests. A difference score is obtained by subtracting one test score from another test score. For example, a student`s IQ Test score and Reading test score. This might be interpreted as the difference between ability and achievement. A difference score is, like the test scores from which it was calculated, unreliable to some extent. Its reliability will be a function of the reliability of the tests used in its computation and the correlation between the scores from the two tests used in its computation as per: Rd = (.5 (Rxx + Ryy) - Rxy)