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Mind the methods of determining minimal important differences: three critical issues to consider
  1. Tahira Devji1,
  2. Alonso Carrasco-Labra2,
  3. Gordon Guyatt1
  1. 1Health Research Methods, Evidence & Impact, McMaster University, Hamilton, Canada
  2. 2Oral and Craniofacial Health Science, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, USA
  1. Correspondence to Dr Tahira Devji, Health Research Methods, Evidence & Impact, McMaster University, Hamilton, Canada; devjits{at}


Objective Clinical trialists, meta-analysts and clinical guideline developers are increasingly using minimal important differences (MIDs) to enhance the interpretability of patient-reported outcome measures (PROMs). Here, we elucidate three critical issues of which MID users should be aware. Improved understanding of MID concepts and awareness of common pitfalls in methodology and reporting will better inform the application of MIDs in clinical research and decision-making.

Methods We conducted a systematic review to inform the development of an inventory of anchor-based MID estimates for PROMs. We searched four electronic databases to identify primary studies empirically calculating an anchor-based MID estimate for any PROM in adolescent or adult populations across all clinical areas. Our findings are based on information from 338 studies reporting 3389 MIDs for 358 PROMs published between 1989 and 2015.

Results We identified three key issues in the MID literature that demand attention. (1) The profusion of terms representing the MID concept adds unnecessary complexity to users’ task in identifying relevant MIDs, requiring meticulous inspection of methodology to ensure estimates offered truly reflect the MID. (2) A multitude of diverse methods for MID estimation that will yield different estimates exist, and whether there are superior options remains unresolved. (3) There are serious issues of incomplete presentation and reporting of key aspects of the design, methodology and results of studies providing anchor-based MIDs, which threatens the optimal use of these estimates for interpretation of intervention effects on PROMs.

Conclusions Although the MID represents a powerful tool for enhancing the interpretability of PROMs, realising its full value will require improved understanding and reporting of its measurement fundamentals.

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  • Contributors The authors are the lead investigators of a programme of research focused on advancing PRO and MID methods. GG is Distinguished Professor in the Departments of Health, Research Methods, Evidence and Impact (HEI), and Medicine at McMaster University, is the Co-Chair of the GRADE Working Group, Co-Convenor of the Cochrane PRO methods group, and an advisor to clinical practice guideline panels worldwide. He developed the initial concept of the MID, first published in 1987, and developed the transition anchor strategy for establishing MIDs, first published in 1989. He has since been active in the health status measurement field, and particularly in interpretation of PROMs. As the senior mentor on our programme of research focused on improving MID and PRO methods, he oversees the conduct of all related projects, provides methodological guidance and facilitates the dissemination of our work through key stakeholder groups such as GRADE, MAGIC, the WHO, the National Institute for Health and Care Excellence, AHRQ, Canadian Agency for Drugs and Technologies in Health (CADTH), and Cochrane. TD and AC-L co-led the systematic review that informed the development of the MID inventory of all published anchor-based MIDs and developed the MID credibility instrument. It is through this prior work that the critical insights presented in this article have emerged. TD wrote the first draft of the manuscript. All authors interpreted the data analysis and critically revised the manuscript. TD is the guarantor.

  • Funding The projects informing the submitted work were funded by the Canadian Institutes of Health Research, Knowledge Synthesis grant number KRS138214.

  • Competing interests No, there are no competing interests.

  • Patient consent for publication Not required.

  • Provenance and peer review Not commissioned; externally peer reviewed.

  • Data availability statement All data relevant to the study are included in the article or uploaded as supplementary information.

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