Reliable change index ($\mathrm{RCI}$)

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 $s\sqrt{2(1-r)}$, givingA 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.

Improvement exceeding a specified measurement-error criterion, with any additional no-deterioration rule made explicit. It differs from crossing a symptom cutoff and from improvement attributable to treatment.

A service outcome combining a reliable improvement criterion with movement from baseline caseness below the relevant clinical cutoffs. The instruments and denominator must be specified; this is not automatically remission established by diagnostic interview.