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Severity Adjusted Effect Size

EffectSize is a standardized method for reporting the magnitude of pre-post change. An effect size of 1 means the patient improved one standard deviation on the outcome questionnaire. The Severity Adjusted Effect Size provides an estimate of effect size after adjusting for differences in case mix, severity of symptoms, point in treatment of first assessment and type of questionnaires.

Simple comparisons of effect sizes may be misleading due to differences in case mix. Also, the severity of symptoms at intake, as measured by the first assessment score, the major predictor of magnitude of change. Patients with severe symptoms will average more improvement than those with mild symptoms, while those with intake scores in the normal range tend to average no improvement on outcome questionnaires. If the first assessment is collected at the first session of treatment, patients average significantly more change than if the first assessment if collected at a mid point in treatment.

Use of the general linear model to calculate residualize gain scores permits comparisons of outcomes after adjusting for differences in case mix, session of first assessment, and questionnaire type. However, while residual gain scores convey information about the difference between actual change compared to predicted change, they convey no information about the total magnitude of change. The severity adjusted effect size conveys information about the magnitude of change while adjusting for differences in case mix.

The severity adjusted effect size is calculated only for patients with intake scores in the clinical range. The method involves calculating the average change score for all cases in the clinical range from the entire population of patients as a reference sample, adding the residualized gain score for each patient to this constant, and then dividing this sum by the standard deviation of the reference sample.

Limiting the calculation of effect size to cases with intake scores in the clinical range has the effect of measuring pre-post change for only those patients with symptoms of sufficient severity that improvement with treatment is expected. This also has the benefit of enabling benchmarking against published research studies, which likewise are conducted using patients with clinical levels of distress.

Effect sizes of 0.8 or larger are considered large, while effect sizes of 0.5 to 0.8 can be considered moderately large. Effect sizes of less than 0.3 are small and might well have occurred without any treatment at all. The ACORN Criteria for Effectiveness (ACE) uses 0.5 effect size as the threshold for "effective".

For more on the practical application of severity adjusted effect size, see ACEOutcomesEvaluationMethod technical specifications.

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