TP. Clinimetric evaluation of the Physical Mobility Scale sup-cinports

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1 2140 ORIGINAL ARTICLE Clinimetric Evaluation of the Physical Mobility Scale Supports Clinicians and Researchers in Residential Aged Care Anna L. Barker, BPhty, MPhty, Jennifer C. Nitz, BPhty, MPhty, PhD, Nancy L. Low Choy, BPhty, MPhty, PhD, Terry P. Haines, BPhty, PhD ABSTRACT. Barker AL, Nitz JC, Low Choy NL, Haines 2008 by the American Congress of Rehabilitation Medi- and the American Academy of Physical Medicine and TP. Clinimetric evaluation of the Physical Mobility Scale sup-cinports clinicians and researchers in residential aged care. ArchRehabilitation Phys Med Rehabil 2008;89: T Objective: To investigate the interrater agreement and the HE PHYSICAL MOBILITY Scale is an ordinal measure internal construct validity of the Physical Mobility Scale, a tool developed by physiotherapists to assess the mobility of routinely used to assess mobility of people living in residential people living in residential aged care. It is a physical performance test that requires the user to observe and score residents aged care. Design: Prospective, multicenter, external validation study. independence and equipment requirements for 9 mobility tasks Setting: Nine residential aged care facilities in Australia. including bed mobility, sitting and standing balance, ambulation, and transfers. 1 Assessment and management of mobility Participants: Residents (N 186). Phase 1 cohort (99 residents; mean age, y); phase 2 cohort (87 residents; impairment is an integral part of clinical management of residents in residential aged care. In this population, mobility is an mean age, y). 2 Interventions: Not applicable. important risk factor for adverse health events such as respiratory tract infections, 3 falls, 4-6 and fractures. 7 Beyond adverse Main Outcome Measures: Kappa statistics, minimal detectable change (MDC 90 ) scores, and Bland-Altman plots were health outcomes for residents, mobility impairment increases used to assess interrater agreement. Scale unidimensionality, the need for physical assistance, which increases risk of staff item hierarchy, and person separation were examined with injury. 8 Accurate mobility assessment informs treatment and Rasch analysis for both cohorts. promotes the use of appropriate equipment and assistance, which Results: Agreement between raters on 6 of the 9 Physicalmay reduce the risk of injuries sustained by residents and staff. Mobility Scale items was high.60). ( The MDC 90 value was Having accurate, reliable, and valid tools to measure resident 4.39 points, and no systematic differences in scores betweenmobility is also essential for clinicians working in residential aged raters were found. The Physical Mobility Scale showed acare to enable resident outcomes to be monitored, and to identify unidimensional structure demonstrated by fit to the Rasch residents at risk of adverse health events. There have been few model in both cohorts (phase 1: , P.16, person comprehensive investigations into the clinimetric properties of separation index 0.96; phase 2: , P.23, person mobility assessments used in residential aged care. separation index 0.96). Standing balance was the most difficult item in both cohorts (phase 1: logit 2.48, SE, 0.16; phase have high levels of interrater agreement, test-retest reliability, The Physical Mobility Scale has previously been reported to 2: logit 2.53, SE, 0.15). The person-item threshold map indicated no floor or ceiling effects in either cohort. 1 and concurrent validity with the Clinical Outcome Variable Scale and the RMI. To date, reliability analysis including Conclusions: The Physical Mobility Scale demonstrated MDC and internal construct validity testing through Rasch good interrater agreement and internal construct validity with analysis for the Physical Mobility Scale have not been reported. The MDC provides clinicians with a value of change good fit to the Rasch model in both cohorts. The comparative results across the 2 cohorts indicate generality of the findings. 9 scoring that must be achieved to overcome measurement error. The Physical Mobility Scale total raw scores can be convertedrasch analysis is based on item response theory and permits to Rasch transformed scores, providing an interval measure ofexamination of the measurement and scaling properties of an mobility. The Physical Mobility Scale may be suited to a range instrument. It provides information about the appropriateness of clinical and research applications in residential aged care. of the summing of item scores to yield a total score (ie, Key Words: Aged; Nursing homes; Outcome assessment representing overall mobility), 10 ceiling and floor effects that (health care); Rehabilitation. can limit ability to measure change over time, and conversion 11 of ordinal to interval-level scores. Ordinal measures, such as the Physical Mobility Scale, provide information about the relative ordering of scores, but not the magnitude of separation From the Division of Physiotherapy, School of Health and Rehabilitation Sciences, between scores. As such, an increase in scoring on an ordinal The University of Queensland (Barker, Nitz) Brisbane; The Northern Clinical Research Centre (Barker) Melbourne; Faculty of Health Sciences and Medicine, Bond University (Low Choy) Robina; Southern Health (Haines) Melbourne; Monash University (Haines) Melbourne, Australia. List of Abbreviations No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated. Supported by the School of Health and Rehabilitation Sciences, University of Queensland, Brisbane, Australia. Reprint requests to Anna L. Barker, BPhty, MPhty, Division of Physiotherapy, School of Health and Rehabilitation Sciences, The University of Queensland, Brisbane, Queensland 4072, Australia, anna.barker@nh.org.au /08/ $34.00/0 doi: /j.apmr BBS MDC MDC 90 PSI RMI Berg Balance Scale minimal detectable change minimal detectable change at 90% confidence interval person separation index Rivermead Mobility Index

2 EVALUATION OF THE PHYSICAL MOBILITY SCALE IN AGED CARE, Barker 2141 measure indicates that a resident s mobility has improved, but it does not provide information about how much improvement has occurred. Interval measures give information on the magnitude of scoring separation, providing information such as percentage change scores. 10 Interval-based measures also offer greater research potential than ordinal measures because parametric statistics can be used. While Rasch analysis has previously been used in the development and validation of several mobility outcome measures, no study has been conduced in the residential aged care setting. 10,12-19 This study aimed to extend the clinimetric evaluation of the Physical Mobility Scale by examining the properties of interrater agreement as measured by the MDC and the ability of the Physical Mobility Scale to fit the Rasch model. METHODS This study used data from a retrospective chart audit of 4 facilities (phase 1) and a prospective validation study conducted in 6 facilities (phase 2). While 1 facility was included in both studies, different people were included in each sample so that the phase 1 cohort was external to the phase 2 cohort. The study was approved by the University of Queensland Medical Research Ethics Committee. Participants and Setting Permanent high-care (nursing home) and low-care (hostel) residents were eligible for inclusion in the study if they had lived at the facility for longer than 12 months. Ninety-nine charts were received for auditing in the phase 1 cohort. Participants in phase 2 were recruited by written informed consent, and 87 residents agreed to participate. Where residents were unable to provide consent, consent was sought from a family member or guardian. Procedure Physical Mobility Scale assessments for the phase 1 cohort were completed by physiotherapists as part of usual care. Physiotherapists independent from the residential aged care facilities (rater 1 and rater 2) completed the assessments for the phase 2 cohort. For the interrater agreement testing, rater 1 conducted the assessment while rater 2 observed and scored the participant on performance. Raters were blind to each other s scoring. Outcome Measure The Physical Mobility Scale is composed of 9 mobility tasks that are scored on a 6-point ordinal scale from least independent (score 0) to independent (score 5) to yield a total score out of 45. A copy of the Physical Mobility Scale tool is available in the article by Nitz et al. 1 Statistical Analysis Data were analyzed using Stata (version 9.2) a for all analyses except the Rasch analysis, which was completed using RUMM2020. b Interrater agreement. Analyses were performed on the first 28 participants recruited in the phase 2 cohort. The statistic was used to assess agreement between raters item scoring of participants. Values of.60 or above were considered evidence of high agreement. 20 The MDC 90 was calculated to assess the error in measurement of the total scores between raters Bland-Altman plots with 95% limits of agreement were constructed to detect systematic bias between rater scores. 24 Internal construct validity testing. Analyses were performed on the phase 1 (n 99) and phase 2 (n 87) cohorts separately to provide information about the generality of the results. Rater 1 scores were used in the Rasch analysis of phase 2 data. Rasch analysis was used to investigate the scaling properties of the Physical Mobility Scale, through probabilistic modelling of item difficulty and person ability on a common linear continuum. 25,26 The properties of unidimensionality (measurement of 1 underlying construct), item hierarchy (the relative difficulty of the items compared with one another), and person separation (the extent to which items distinguish between different levels of mobility) were examined. 27 Overall fit of the Physical Mobility Scale to the Rasch model was determined by the item-fit and person-fit statistics. Item-trait interaction quantifies the fit of the observed data to the predicted model; for instance, a mean of 0 and SD of 1 indicate perfect fit to the model. 28 The chi-square statistics for the overall and individual item-trait interactions should show no significant deviations from the model expectation, confirming scale unidimensionality. 28 Confirming unidimensionality validates the summing of item scores to yield an overall score to measure the underlying construct. 10 Residual fit statistics provide additional information about the unidimensionality of the measure. The residuals are the standardized person-item differences between the observed data and what is expected by the model for every person s response to every item. 27 Item residual statistics should range between 2.5 and ,28 Item fit residuals of greater than 2.5 may indicate the measurement of another construct by that item. If several items are found to have fit residuals that exceeded 2.5, these items may be redundant. Item logit (log odds of the probability) scores were calculated to examine the hierarchy of the scale items. More difficult items have higher logit values, while easier items have lower values. This facilitates the identification of items that are easy enough for residents with low levels of mobility to perform and items that are challenging for residents with high levels of mobility. Person-item threshold maps, displaying person ability and item difficulty on a common logit scale, were constructed to detect floor or ceiling effects. The PSI was calculated as a measure of the scale agreement. The PSI indicates how well the items of the scale separate the participants in the sample. The PSI should be above.70 to allow the scale to differentiate at least 2 strata of resident mobility. 28 Cronbach was used to Table 1: Demographics and Characteristics by Cohort Characteristics Phase 1 n 99 Phase 2 n 87 Age Sex* Women 72 (72.73) 49 (58.33) High-level (nursing home) care* 79 (79.80) 77 (88.51) Diagnosis* Dementia 38 (38.38) 44 (50.57) Osteoporosis 16 (16.16) 20 (22.99) Depression 16 (16.16) 16 (18.39) Diabetes mellitus 14 (14.14) 10 (11.49) Previous fractured neck of femur 14 (14.14) 10 (11.49) Mobility Nonambulant 36 (36.36) 16 (18.39) Independent ambulation 33 (33.33) 18 (20.69) Duration of observation period (y) NOTE. Values are n (%) or mean SD. *Frequency (percentage). Up to 4 diagnoses were recorded for each resident. Diagnoses were as recorded on the medical summary sheet by the resident s treating doctor.

3 2142 EVALUATION OF THE PHYSICAL MOBILITY SCALE IN AGED CARE, Barker Table 2: Interrater Agreement and Rasch Model Fit Statistics by Physical Mobility Scale Item and Cohort PMS Item (SE) Item Difficulty Logits (SE) Item Fit 2 (P) Item Fit Residuals Phase n 28 n 99 n 87 n 99 n 87 n 99 n 87 Supine to left side-lie 0.60 (0.10) 0.45 (0.15) 0.44 (0.14) 1.88 (0.39) 4.24 (0.12) Supine to left side-lie 0.75 (0.11) 0.40 (0.14) 0.87 (0.15) 3.66 (0.16) 1.69 (0.43) Supine to sit 0.80 (0.10) 1.13 (0.16) 2.46 (0.16) 2.35 (0.31) 3.33 (0.19) Sitting balance 0.47 (0.12) 1.43 (0.16) 1.61 (0.17) 1.87 (0.39) 1.75 (0.42) Sit to stand 0.67 (0.10) 0.82 (0.18) 0.96 (0.14) 4.43 (0.11) 3.71 (0.16) Stand to sit 0.46 (0.09) 0.91 (0.17) 0.75 (0.17) 2.49 (0.29) 3.12 (0.21) Standing balance 0.61 (0.09) 2.48 (0.16) 2.53 (0.15) 3.83 (0.15) 0.44 (0.80) Transfers 0.62 (0.10) 0.47 (0.15) 0.15 (0.16) 1.24 (0.54) 0.50 (0.78) Mobility 0.59 (0.09) 0.34 (0.14) 0.98 (0.15) 2.14 (0.34) 3.21 (0.20) NOTE. High positive logit values represent difficult items, while low negative values represent easy items. Nonsignificant (P.05) item fit 2 values indicate fit to the Rasch model and confirm unidimensionality. Abbreviations: PMS, Physical Mobility Scale. measure the correlation between items within the Physical Mobility Scale, representing a measure of internal consistency. Alpha values greater than.70 were deemed acceptable. Once fit to the Rasch model was confirmed, raw Physical Mobility Scale total scores were transformed into interval-level scores. RESULTS Table 1 shows the demographic characteristics of participants in the study. Residents in the phase 1 cohort were more likely to be women (Pearson ; P.019) and nonambulant (Pearson ; P.006). No other significant differences between cohorts were found (P.05). No participants withdrew from the study, and no adverse events attributable to the study assessments were reported. The MDC 90 value was 4.39 points. Agreement between raters on 6 of the 9 items was high (.60) (table 2). The Bland-Altman plot (fig 1) shows that the mean differences between rates scores was small (mean difference,.214; 95% confidence interval, 1.28 to 0.85), indicating no systematic differences. Table 2 summarizes the results of the Rasch analysis. Analysis of individual item fit statistics reveal that all 9 Physical Mobility Scale items did not deviate significantly (P.05) from the Rasch model, confirming unidimensionality and the appropriateness of summing of item scores to provide an overall measure of mobility. Further supporting unidimensionality, item-trait interaction statistics identified no significant deviations from the Rasch model in both cohorts (phase 1: , P.16; and phase 2: , P.23). These results indicate that resident performance scores were consistent with the order of item difficulty higher scoring residents were able to perform more difficult items. The residual mean value for the items was 0 in both cohorts, with an SD of 1.21 in the phase 1, and 1.52 in the phase 2 cohorts, further providing support of unidimensionality. The PSI in both cohorts (.96) indicated that the participants were well targeted to the items, 29 and that the Physical Mobility Scale was very effective in discriminating differing levels of mobility across the sample. Standing balance was the most difficult task in both cohorts, and supine to sit and sitting balance the easiest (see table 2). There was a lack of consistency between the item difficulty parameters across cohorts for the supine to sit, transfers, and mobility items. The person-item threshold maps (fig 2) indicated that the range of item difficulty was comprehensive and well matched to person ability, showing no floor or ceiling effects. Cronbach Fig 1. Bland-Altman plot of interrater agreement. Abbreviations: LOA, limits of agreement; PMS, Physical Mobility Scale.

4 EVALUATION OF THE PHYSICAL MOBILITY SCALE IN AGED CARE, Barker 2143 Fig 2. Person-item threshold maps by cohort. Abbreviation: Freq, frequency. was.98 in the phase 1 cohort and.97 in the phase 2 cohort, indicating high internal consistency. Interitem correlations were also high in phase 1 (mean,.80.09; range,.62.96) and phase 2 (mean,.76.07; range,.68.85). Raw ordinal-level Physical Mobility Scale total scores were transformed to interval-level Rasch transformed scores (appendix 1). DISCUSSION This study provides new evidence of the Physical Mobility Scale MDC 90 scores and internal construct validity. The Physical Mobility Scale was found to have a small error in measurement and good individual item and overall fit to the Rasch model. These results support continued clinical and research use of the Physical Mobility Scale, and validate the Physical Mobility Scale as an interval measure of mobility in residential aged care. The Physical Mobility Scale is the first mobility measure to be validated by Rasch analysis in the residential aged care setting. The Rasch transformed interval scores yield a total Physical Mobility Scale score out of 100, because such change scores can be interpreted as a percentage change in mobility. 10 The interval Physical Mobility Scale score also offers advanced research application because parametric statistical analyses can be employed. In addition to these attributes, the Physical Mobility Scale has several benefits over existing mobility measures used in residential aged care. Over half of high-care (nursing home) residents are nonambulant 30 and consequently are unable to perform assessments such as the Timed Up & Go test 30 and the 10-foot walk test, 5 and are unable to complete many items on the BBS 30 and RMI, 31 resulting in floor effects in these measures. The Physical Mobility Scale (see fig 2) was found not to have floor or ceiling effects. The prevalence of dementia in residential aged care is high, generally greater than 30%. 32 Over 50% of participants in the phase 2 of this study had a documented diagnosis of dementia (see table 1). The BBS 30 and the RMI 31 require residents to follow instructions to perform the assessed mobility tasks. People with cognitive impairment may have difficulty following these instructions and so may be unable to complete the full assessment. The Physical Mobility Scale includes mobility tasks that are commonly performed by residents in everyday activities, as opposed to more clinically oriented tests such as the Timed Up & Go test, BBS, and RMI. Accordingly, the Physical Mobility Scale assessment can be performed through observation of the residents moving in their everyday activities without the need for the residents to follow multistep instructions. Measures such as timed chair stands and the 10-foot walk test are more limited assessment tasks because they provide information about only 1 component of mobility, omitting information on performance of activities such as bed mobility, ambulation (for timed chair stands), or chair mobility and sitting balance (for the 10-foot walk test). Further, they do not provide information about the level of assistance and equipment that a resident requires to

5 2144 EVALUATION OF THE PHYSICAL MOBILITY SCALE IN AGED CARE, Barker perform mobility tasks safely. These tools therefore do not facilitate the prevention of handling injuries that could be sustained by residents and staff. In contrast, the Physical Mobility Scale includes a broad range of commonly performed resident mobility tasks, and identifies staff and equipment assistance requirements. Consequently, the Physical Mobility Scale offers a valid and reliable method for measuring and communicating information to staff about the level of assistance and equipment that a resident requires to perform mobility tasks safely. The MDC 90 score obtained means that a change in Physical Mobility Scale raw score of greater than 5 points is required before the change in scoring can be interpreted as not being error in measurement. These results suggest that the Physical Mobility Scale is sufficiently sensitive to detect change in a resident s mobility. The items sitting balance, stand to sit, and mobility demonstrated only moderate interrater agreement, while the remaining 6 items were found to have high levels of interrater agreement. Lower agreement may be reflective of the subjective wording of these items. However, the statistics of.46 through.59 obtained in this study, and intraclass correlation coefficients of.68 through.88 from the past study for these 3 items, 1 represent clinically useful levels of agreement, so review and modification of these items is not of crucial importance. Study Limitations Several limitations of the study need to be acknowledged. First, although the concordance of estimates across cohorts provides promising evidence of the generality of the results, the sample sizes used were relatively small for Rasch analysis. This may affect the precision of the estimates of the Rasch model, particularly the logit scores of item difficulty. As such, the item difficulty results should not be overinterpreted. A large-scale study is still required to establish normative values for Physical Mobility Scale scores and to confirm the findings of the current study. Second, the use of concurrent observational assessment by the 2 raters means that the reliability of application of the tools by the raters was not assessed. CONCLUSIONS This study provides important insights into the clinimetric properties of a commonly used mobility assessment tool in residential aged care. The Physical Mobility Scale demonstrated good interrater agreement and internal construct validity including good individual item and overall fit to the Rasch model. The Physical Mobility Scale total raw scores can be converted to Rasch transformed scores, providing an interval measure of mobility. This study therefore provides evidence that the Physical Mobility Scale is a valid and reliable measure of mobility for people living in residential aged care, and is suited to a range of clinical and research applications. APPENDIX 1: RAW SCORE TO RASCH SCORE CONVERSION TABLE Phase 1 n 99 Raw Logit Rasch Transformed APPENDIX 1: RAW SCORE TO RASCH SCORE CONVERSION TABLE (Cont d) Phase 1 n 99 Raw Logit Rasch Transformed References 1. Nitz JC, Hourigan SR, Brown A. Measuring mobility in frail older people: reliability and validity of the Physical Mobility Scale. Australas J Ageing 2006;25: Saliba D, Solomon D, Rubenstein L, et al. Quality indicators for the management of medical conditions in nursing home residents. J Am Med Dir Assoc 2005;6(Suppl 3):S Binder EF, Kruse RL, Sherman AK, et al. Predictors of short-term functional decline in survivors of nursing home-acquired lower respiratory tract infection. J Gerontol A Biol Sci Med Sci 2003; 58: Capezuti E, Boltz M, Renz S, Hoffman D, Norman RG. Nursing home involuntary relocation: clinical outcomes and perceptions of residents and families. J Am Med Dir Assoc 2006;7: Thapa PB, Gideon P, Brockman KG, Fought RL, Ray WA. Clinical and biomechanical measures of balance as fall predictors in ambulatory nursing home residents. J Gerontol A Biol Sci Med Sci 1996;51:M

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