Signature
MID_between = d_M - d_S
| Inputs | Definition | Unit |
|---|---|---|
d_M | A little better for improvement, a little worse for deterioration | score points |
d_S | Unit: score points | — |
MID_between | Unit: score points | — |
|---|
Function
Anchor-based thresholds for a minimal clinically important difference and their use for responder shares and trial size
Maps a meaningful step on an external anchor, usually a patient's global rating of change, onto the score scale of an outcome measure. The between-group estimate subtracts the mean change of patients reporting no change from that of patients reporting a small change; the ROC cut-off chooses the change score that best separates anchor-improved patients from the rest; the 95 per cent limit cut-off sits above almost all unchanged patients. A threshold then gives the share of patients reaching it in each arm, and a between-group difference sets the target difference of a sample size calculation. Distribution-based yardsticks are HE-FM-DMID-001 to HE-FM-DMID-003 on the Distribution-Based MID page. Notation follows the Minimal Clinically Important Difference article.
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Implementations
Excel
Between-group minimal important difference from named cells
With MeanChgMin and MeanChgSame named, the formula returns the between-group estimate, held in MIDBetween.
=MeanChgMin-MeanChgSame
Assumptions
Anchor correlated with the change score
The anchor is valid for the score: Terwee and colleagues ask for a correlation of at least 0.30 between the change score and the anchor, and the credibility instrument of Devji and colleagues treats at least 0.5 as moderate to high; the article's example has 0.52.
Both anchor groups from the same cohort and interval
The two groups were measured with the same instrument over the same interval, and the category that counts as minimal on the anchor is chosen before the analysis.
Worked examples
Validation cohort with mean changes of 8.0 and 2.0 points
Patients rating themselves a little better improved by 8.0 points on average and those rating themselves about the same by 2.0, so the between-group estimate is 8.0 minus 2.0, or 6.0 points, as in the article; the mean change method alone would give 8.0.
d_M = 8; d_S = 2; MID_between = 6
Deterioration in the same cohort
Patients rating themselves a little worse changed by minus 5.0 points, so the between-group estimate for deterioration is minus 7.0 points, larger in size than the 6.0 for improvement (computed here for illustration).
d_M = -5; d_S = 2; MID_between = -7
Common errors
Using the within-person mean change for a difference between arms
Terwee and colleagues point out that a difference between patients reporting a little better and about the same is a minimal important difference between groups, not a within-person minimal important change; in the article's example these are 6.0 and 8.0 points.
Ignoring the choice of anchor category
De Vet and colleagues show that the estimate can differ by a factor of about 2.5 depending on whether the first category above no change or moderate to much improvement is chosen, and that the choice cannot rest on statistical grounds.
Sources
Terwee and colleagues on MID between groups and MIC within persons
Terwee CB, Peipert JD, Chapman R, Lai JS, Terluin B, Cella D, Griffiths P, Mokkink LB. Minimal important change (MIC): a conceptual clarification and systematic review of MIC estimates of PROMIS measures. Quality of Life Research. 2021;30(10):2729-2754. doi:10.1007/s11136-021-02925-y (full text read). Part 1: a difference between patients who reported to be a little better and those who reported to be about the same refers to a minimal important difference, not a minimal important within-person change. Box 1: the correlation between the change score and the anchor should be at least 0.30 to assume validity of the anchor.
Jaeschke and colleagues on comparing score changes with global ratings
Jaeschke R, Singer J, Guyatt GH. Measurement of health status: ascertaining the minimal clinically important difference. Controlled Clinical Trials. 1989;10(4):407-415. doi:10.1016/0197-2456(89)90005-6 (abstract read). Abstract: the significance of changes in quality of life scores was elucidated by comparing them with global ratings of change; in three studies the MCID was represented by a mean change of approximately 0.5 per item on a seven-point scale.
De Vet and colleagues on the anchor category and the size of the MIC
de Vet HC, Terwee CB, Ostelo RW, Beckerman H, Knol DL, Bouter LM. Minimal changes in health status questionnaires: distinction between minimally detectable change and minimally important change. Health and Quality of Life Outcomes. 2006;4:54. doi:10.1186/1477-7525-4-54 (full text read). Section on the definition of important change on the anchor: in studies requiring moderate or much improvement the MIC corresponds to about 2.5 times the SEM, against close to one SEM for the first category above no change; the choice cannot be based on statistical characteristics.
Devji and colleagues on the correlation of the anchor with the change score
Devji T, Carrasco-Labra A, Qasim A, Phillips M, Johnston BC, Devasenapathy N, et al. Evaluating the credibility of anchor based estimates of minimal important differences for patient reported outcomes: instrument development and reliability study. BMJ. 2020;369:m1714. doi:10.1136/bmj.m1714 (full text read). Explanation of core item 3: a moderate to high correlation, at least 0.5, suggests the validity of the anchor.
Canonical Identity
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