An investigation of the measurement properties of the de Morton Mobility Index for measuring mobility capacity in hospital patients with Parkinson’s disease

OBJECTIVE: To examine the measurement properties of the de Morton Mobility Index (DEMMI), a performance-based clinical outcome assessment of mobility capacity, in hospital patients with Parkinson’s disease. DESIGN: Cross-sectional study. PARTICIPANTS: Hospital patients with Parkinson’s disease. MAIN OUTCOME MEASURE(S): Structural validity and unidimensionality (Rasch analysis), construct validity, internal consistency reliability, and inter-rater reliability of the de Morton Mobility Index (scale range: 0–100 points) were established. The minimal detectable change, the 95% limits of agreement and possible floor and ceiling effects were calculated to indicate interpretability. RESULTS: We analysed validity (n = 100; mean age: 70 years; 71% male) and reliability (n = 47; mean age: 71 years; 68% male) in two samples. The mean Hoehn and Yahr stage was 3.2 and the mean disease duration was 12 years in both samples. Rasch analysis indicated unidimensionality with an overall fit to the model (chi-square = 21.49, P = 0.122). Seventy-three percent of hypotheses on construct validity were confirmed. Internal consistency reliability (Cronbach’s alpha = 0.91) and inter-rater reliability (intraclass correlation coefficient = 0.88; 95% confidence interval: 0.80 to 0.93) were sufficient. The minimal detectable change with 90% confidence was 17.5 points and the limits of agreement were 31%. No floor or ceiling effects were observed. The mean administration time was 6.6 minutes. CONCLUSION: This study provides evidence of unidimensionality, sufficient internal consistency reliability, inter-rater reliability, construct validity, and feasibility of the de Morton Mobility Index in hospital patients with Parkinson’s disease. TRIAL REGISTRATION: German Clinical Trials Register (DRKS00004681). Registered May 6, 2013.