= Reliable change index
{title2=$\mathrm{RCI}$}
= RCI
{c}
{synonym}
A change score scaled by its expected measurement error. With equal marginal standard deviation $s$, reliability $r$ and independent measurement errors at the two assessments, the difference has error standard deviation $s\sqrt{2(1-r)}$, giving
$$
\mathrm{RCI}=\frac{X_{\mathrm{post}}-X_{\mathrm{pre}}}{s\sqrt{2(1-r)}}.
$$
A conventional normal-error criterion compares its magnitude with 1.96. The assumptions and instrument-specific reliability matter; reliable change is not itself <diagnostic remission> or proof that treatment caused the change.
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