Although not consistently superior, the absolute approach to framing the minimally important difference has advantages over the relative approach
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OBJECTIVES: Using studies that established minimal important difference (MID) using anchor-based methods, we set out to address the relative merits of absolute and relative changes in establishing an instrument's MID. STUDY DESIGN AND SETTING: 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 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. CONCLUSION: Although we found no consistent superiority of either approach to establishing the MID, when differences existed they usually favored 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.
