# A modified effect size for assessing the magnitude of change

Guyatt's responsiveness index (Guyatt et al, 1987) is an effect size for comparing one or more treatment groups with a control group. Lancaster (2009) suggests also using this to compare subsequent time points to baseline.

The formula for Guyatt’s responsiveness index is the [minimally important difference in the response/sqrt(2)] multiplied by the MSE where MSE is the mean squared error of the response obtained from an analysis of variance model that examines repeated observations of the measure in the controls. Alternatively, if there are only 2 observations of the measure, MSE is the standard deviation (SD) of the individual change scores in the reference group (e.g., controls or baseline). Guyatt’s index is calculated as the ratio of the mean change of patients in the treatment group divided by the SD of change in the control group.

Reference

Guyatt G, Walter S, Norman G. (1987). Measuring change over time: assessing the usefulness of evaluative instruments. *J Chronic Dis* **40** 171–8.