Though not consistently superior, the absolute approach to framing the minimally important difference has advantages over the relative approach

Article type
Authors
Zhang Y1, Zhang S1, Thabane L1, Furukawa TA2, Johnston BC3, Guyatt GH4
1Department of Clinical Epidemiology and Biostatistics, McMaster University, Canada
2Department of Health Promotion and Human Behavior, Kyoto University School of Public Health, Kyoto, Japan
3Department of Anaesthesia and Pain Medicine, The Hospital for Sick Children, University of Toronto; Institute of Health Policy, Canada
4Department of Clinical Epidemiology and Biostatistics, and Medicine, McMaster University, Canada
Abstract
Background: Whether the minimal important difference (MID) is best framed using an absolute change or a relative change in score when interpreting patient-reported outcomes remains uncertain.
Objectives: Using studies that established MID using anchor-based methods, we set out to address the relative merits of absolute and relative changes in establishing an instrument’s MID.
Methods: In seven data sets, we calculated correlations between global change ratings and absolute and relative score changes and conducted meta-analyses. We considered that the measure with the higher correlation represented the more valid approach.
Results: The meta-analyses showed no significant difference between pooled correlations of absolute and relative difference on a health-related quality of life instrument with global transition scores of symptoms, emotional function, physical function, and cognitive function. In four of five domains there was at least one study in which the absolute was significantly superior to the relative; in one of these four, one study showed statistically significant superior performance of the relative. In an analysis restricted to patients with low baseline scores for the domain of cognitive function, the relative approach showed higher correlation with global rating than did the absolute approach.
Conclusions: Although we found no consistent superiority of either approach to establishing the MID, when differences existed they usually favoured the absolute, which also has advantages of simplicity and ease of pooling across studies. Researchers may consider the absolute as a default, but also compare both methods on an instrument by instrument basis.