Background Although less likely to be reported in clinical trials than expressions of the statistical significance of differences in outcomes, whether or not a treatment has delivered a specified minimum clinically important difference (MCID) is also relevant to patients and their caregivers and doctors. Many dementia treatment randomised controlled trials (RCTs) have not reported MCIDs and, where they have been done, observed differences have not reached these. Methods As part of the development of the Statistical Analysis Plan for the DOMINO trial, investigators met to consider expert opinion- and distribution-based values for the MCID and triangulated these to provide appropriate values for three outcome measures, the Standardised Mini-mental State Examination (sMMSE), Bristol Activities of Daily Living Scale (BADLS) and Neuropsychiatric Inventory (NPI). Only standard deviations (SD) were presented to investigators who remained blind to treatment allocation. Results Adoption of values for MCIDs based upon 0.4 of the SD of the change in score from baseline on the sMMSE, BADLS and NPI in the first 127 participants to complete DOMINO yielded MCIDs of 1.4 points for sMMSE, 3.5 for BADLS and 8.0 for NPI. Conclusions Reference to MCIDs is important for the full interpretation of the results of dementia trials and those conducting such trials should be open about the way in which they have determined and chosen their values for the MCIDs
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