The unreliable change of reliable change indices

The classic method for assessment of reliable change, in 1991 re-introduced as Jacobsonʹs RC, can be characterized as a confidence interval method. In recent years, several RC indices have been proposed using Kelleyʹs (1947) (Kelley, T. L. (1947). Fundamentals of statistics. Cambridge: Harvard University Press) formula for estimating true change. In these proposals, interval estimation and confidence intervals are mixed up, which leads to unjustified probability statements. When Kelleyʹs estimate is correctly expanded into a normal distributed statistic, the classic approach reveals itself as a large sample approximation of a properly constructed RCI based on Kelleyʹs formula. Researchers should continue using the classic approach for the determination of reliable change.