Modified equations for the validity and reliability of difference scores that describe applied testing situations are examined. This examination reveals that simple gain scores can be more useful in research than has commonly been believed. Simple gain scores are neither inherently unreliable nor lack predictive validity. (SLD)

Changes, like single scores, are demonstrated to be valid and reliable and potentially predictable by independent variables. Change and growth measures determined from individual examinees' test scores indicated excellent predictive value. (Author/PN)

The critiques by L. Collins and L. Humphreys in this issue illustrate problems with the use of gain scores. Collins' examples show that familiar formulas for the reliability of differences do not reflect the precision of measures of change. Additional examples demonstrate flaws in the conventional approach to reliability. (SLD)