Clinimetric Evaluation of the Physical Mobility Scale Supports Clinicians and Researchers in Residential Aged Care

Abstract 

Barker AL, Nitz JC, Low Choy NL, Haines TP. Clinimetric evaluation of the Physical Mobility Scale supports clinicians and researchers in residential aged care.

Objective

To investigate the interrater agreement and the internal construct validity of the Physical Mobility Scale, a tool routinely used to assess mobility of people living in residential aged care.

Design

Prospective, multicenter, external validation study.

Setting

Nine residential aged care facilities in Australia.

Participants

Residents (N=186). Phase 1 cohort (99 residents; mean age, 85.22±5.1y); phase 2 cohort (87 residents; mean age, 81.59±10.69y).

Interventions

Not applicable.

Main Outcome Measures

Kappa statistics, minimal detectable change (MDC₉₀) scores, and Bland-Altman plots were used to assess interrater agreement. Scale unidimensionality, item hierarchy, and person separation were examined with Rasch analysis for both cohorts.

Results

Agreement between raters on 6 of the 9 Physical Mobility Scale items was high (κ>.60). The MDC₉₀ value was 4.39 points, and no systematic differences in scores between raters were found. The Physical Mobility Scale showed a unidimensional structure demonstrated by fit to the Rasch model in both cohorts (phase 1: χ²=23.90, P=.16, person separation index=0.96; phase 2: χ²=22.00, P=.23, person separation index=0.96). Standing balance was the most difficult item in both cohorts (phase 1: logit=2.48, SE, 0.16; phase 2: logit=2.53, SE, 0.15). The person-item threshold map indicated no floor or ceiling effects in either cohort.

Conclusions

The Physical Mobility Scale demonstrated good interrater agreement and internal construct validity with good fit to the Rasch model in both cohorts. The comparative results across the 2 cohorts indicate generality of the findings. The Physical Mobility Scale total raw scores can be converted to Rasch transformed scores, providing an interval measure of mobility. The Physical Mobility Scale may be suited to a range of clinical and research applications in residential aged care.

Key Words: Aged, Nursing homes, Outcome assessment (health care), Rehabilitation

List of Abbreviations: BBS, Berg Balance Scale, MDC, minimal detectable change, MDC₉₀, minimal detectable change at 90% confidence interval, PSI, person separation index, RMI, Rivermead Mobility Index