Interpretation of changes in health-related quality of life the remarkable universality of half a standard deviation

Geoffrey R. Norman, Jeff A. Sloan, Kathleen W. Wyrwich

Research output: Contribution to journalArticlepeer-review

2671 Scopus citations


BACKGROUND. A number of studies have computed the minimally important difference (MID) for health-related quality of life instruments. OBJECTIVE. To determine whether there is consistency in the magnitude of MID estimates from different instruments. METHODS. We conducted a systematic review of the literature to identify studies that computed an MID and contained sufficient information to compute an effect size (ES). Thirty-eight studies fulfilled the criteria, resulting in 62 ESs. RESULTS. For all but 6 studies, the MID estimates were close to one half a SD (mean = 0.495, SD = 0.155). There was no consistent relationship with factors such as disease-specific or generic instrument or the number of response options. Negative changes were not associated with larger ESs. Population-based estimation procedures and brief follow-up were associated with smaller ESs, and acute conditions with larger ESs. An explanation for this consistency is that research in psychology has shown that the limit of people's ability to discriminate over a wide range of tasks is approximately 1 part in 7, which is very close to half a SD. CONCLUSION. In most circumstances, the threshold of discrimination for changes in health-related quality of life for chronic diseases appears to be approximately half a SD.

Original languageEnglish (US)
Pages (from-to)582-592
Number of pages11
JournalMedical care
Issue number5
StatePublished - May 2003


  • Effect size
  • Interpretation
  • MID
  • Quality of life
  • Threshold

ASJC Scopus subject areas

  • Public Health, Environmental and Occupational Health


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