The critiques of Collins (1996) and Humphreys
(1996) certainly throw light on properties of gain scores
and difference scores that have led to controversies in
the past. Collins’ examples reveal that familiar formulas
for the reliability of differences do not adequately reflect
the precision of measures of change, because they do
not allow for intraindividual change. Some additional
examples are provided here, and a similar argument is
applied to the reliability of a single test. As Collins implies,
these arguments indeed disclose flaws, not only in
the conventional approach to the reliability of gains and
differences, but also in the basic concept of reliability in
classical test theory. Index terms: change scores, classical
test theory, difference scores, gain scores, intraindividual
differences, measurement of growth, reliability,
test theory, validity.
Williams, Richard H & Zimerman, Donald W. (1996). Are simple gain scores obsolete: Commentary on the commentaries of Collins and Humphreys. Applied Psychological Measurement, 20, 295-297. doi:10.1177/014662169602000310
Williams, Richard H.; Zimmerman, Donald W..
Commentary on the Commentaries of Collins and Humphreys.
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