A note on the correlation of gain scores and achievement level
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The negative correlation between gain score and initial status is one of the classical dilemmas in the measurement of change. A simple but efficient method is proposed to get valid information about the relationship between change and the level of achievement. After finishing and submitting this paper, the author became aware of the fact that the proposed rotation of the 2-dimensional space defined by pre- and post-test has already been presented and discussed by P.D. Oltham 50 years ago. However, there is no reason to withdraw the paper, since the majority of empirical researchers still try to derive correct results on the relationship of level and growth without the simple but efficient method of rotating the data space by 45 degrees. Addressed to these researchers, I would say “it’s time to make a change”.