Reliable and clinically significant change based on the Health of the Nation Outcome Scales

•Clinicians need to monitor progress or decline of individual patients during treatment.•The Health of the Nation Outcome Scales can be used to monitor treatment outcome.•A statistical sound method to calculate reliable and clinical significant change (RCSC) is essential.•In heterogeneous samples results of this method are often conceived as disappointing.•Proposed adjustments to the (RCSC) method make it a fair and realistic outcome assessment. To evaluate treatment outcomes of individual patients based on clinician-rated instruments, the assessment of reliable and clinically significant change (RCSC) is essential. In heterogeneous samples, RCSC underestimates treatment outcome. Therefore, the Reliable Change Index (RCI) was adjusted by a stratification into subsamples. This method was tested on the Health of the Nation Outcome Scales for Children and Adolescents (HoNOSCA) ratings (n = 12,547) at admission and discharge of youths (age 6–18 years) treated in ten psychiatric institutions. Based on the test-retest reliability of a subsample (n = 397), the RCI was calculated for three severity groups (“very severe,” “moderately severe,” and “subclinical/mild”). Individuals who accomplished reliable change during treatment and moved to a subclinical score were classified as recovered. Using the traditional RCSC calculation, the large majority (75.1%) of the sample would be considered as unchanged, 2.9% as deteriorated, 18.9% as improved, and 3.1% as recovered. Using RCI cutoff points based on the severity ratings at admission results in a more representative distribution of outcome groups, where 54.6% of the sample was stable, 7.5% worsened, 21.6% improved, and 16.3% recovered. This methodological framework for calculating RCSC for heterogeneous populations is applicable for all HoNOS instruments, making it very useful for mental health professionals.