Reliability and Validity of the Telephone Administration of the Wheelchair Outcome Measure (WhOM) for Middle-Aged and Older Users of Power Mobility Devices

Abstract

Objective

To examine the measurement properties of the telephone administration of the Wheelchair Outcome Measure (WhOM).

Subjects

Power mobility device users aged 50–89 years.

Methods

Two independent cohorts were recruited: 1) a prospective cohort (n=40) to estimate test-retest reliability and to determine the applicability of the telephone format, and 2) a cross-sectional cohort to examine construct validity with 3 groups: i) people waiting for a first power mobility device (n=44); ii) new users (n=35;1–6 months), and iii) long-term users (n=39;12–18 months).

Results

The tool demonstrated good test-retest reliability (ICCs .77–1.00), was administered in 10.9 minutes (SD=5.2) and was practical to use over the telephone. The validity testing showed moderate correlations with the Quebec User Evaluation of Satisfaction with Technology (QUEST 2.0, r_S=.36–.45) and the Psychosocial Impact of Assistive Devices Scale (PIADS-10, r_S=31–.43). WhOM scores could discriminate users based on duration of use (p<.001) and device type (power wheelchair vs scooter, p<.05).

Conclusions

The WhOM is a stable, valid and applicable measure for telephone administration with older power mobility device users. It is moderately linked to satisfaction with the device and to the psychosocial impact of the device, and therefore complements rather than replaces those measures.

The eligibility criteria for power mobility devices (PMDs), such as power wheelchairs and scooters, are a contentious issue in many jurisdictions (1–4) and there is a growing need for sound evidence on the outcomes of these assistive technologies (ATs). Mobility-related subsidy programs for ATs are being challenged by the aging of the population (5–7). Adults aged over 50 years are the most prevalent wheelchairs users (8, 9) and it is estimated that PMD use is 3.5 times more frequent after the age of 65 (5). However, very few studies have addressed the impact of PMDs on the lives of older adults (10) and they have generally relied on outcome measures with very little evidence of reliability and validity for this population (11, 12).

One of the fundamental goals of PMD provision is to improve “functioning”, that is to maximize the potential use of body functions (body level), to increase the capacity to perform activities (activity level) and ultimately to allow an expansion of participation in valued life roles (participation level) (13). A critical review of measurement tools designed for wheelchair users indicated that most of these tools assess only mobility activities, and emphasize a normative evaluation of capacities of the users in standardized settings (11). To appreciate the full impact of PMDs at the participation level, the range of outcome domains has to be expanded beyond mobility activities. Moreover, the input of the user is crucial since there is no universal standard for successful participation in valued activities and life roles (14, 15). Some wheelchair user-specific questionnaires have been developed to assess self-perceived wheelchair skills (16) and function (17) related to wheelchair/scooter use. Moreover, an exhaustive participation measure for individuals with various mobility limitations is available (18). However, these tools were not designed to take into account the participation aspirations of the wheelchair user, and whether or not significant activities and valued social roles were effectively enabled by the device.

The Wheelchair Outcome Measure (WhOM) (19) is an individualized, goal-oriented measure of outcome related to wheelchair intervention. Wheelchair intervention is defined in broad terms and covers either a new wheelchair prescription, wheelchair renewal, adjustments to the device/environment or additional training of the user. The WhOM is designed to identify desired outcomes at the participation level, but also some body structure and function items as defined by the International Classification on Functioning, Disability and Health (13). The tool measures importance and satisfaction with a range of self-selected, wheelchair-related activities. The WhOM was found to be reliable and valid with a population of young adults with spinal cord injuries who used manual and power wheelchairs (20). Test-retest and inter-rater reliability intraclass correlation coefficients (ICCs) estimates were substantial during face-to-face interviews (ICCs>.90), and construct validity was supported by moderate associations (.33<r<.66) with a generic participation measure, as well as with satisfaction with assistive technologies (20).

As part of a larger study, the WhOM was adapted and translated into French in order to conduct telephone interviews with middle-aged and older adults. Telephone questionnaires are less time-consuming than face-to-face interviews (21) and provide easier access to broad geographic areas at a lower cost. Since reliability and validity are context and population-specific attributes (22), the translation of a measurement tool and the modification of the assessment format require additional psychometric testing to verify the equivalence of the measure.

The objective of this study was to examine the properties of the French version of the WhOM in relation to reliability and applicability for a telephone administration, as well as construct validity of the WhOM for French and English speaking middle-aged and older PMD users. Such information would confirm if the measurement properties of the tool are adequate for telephone interviews with PMD users and extend its use for research and clinical purposes with French or English-speaking populations as well as older adults.

Methods

Study design

This study involved two independent cohorts of PMD users: 1) a French-speaking prospective cohort to estimate reliability, as well as the applicability of the telephone format (reliability sample), and 2) a French or English-speaking cross-sectional cohort to examine construct validity (validity sample). A test-retest approach was used to address reliability. This approach estimates the stability of the responses by repeating the questionnaire during a period where no changes are expected for the construct under study (23). Applicability refers to context- and population-specific pragmatic qualities of an assessment tool (24). During the telephone interview, applicability was assessed by coding the burden of assessment for the participant and the examiner, and the identification of floor or ceiling effects. In the absence of a gold standard, construct validity was examined through convergent and discriminant validity testing. Convergent validity estimated the degree of association with the satisfaction with the device and with the psychosocial impact for PMD users. Discriminant validity testing was used to verify if the WhOM could distinguish between users of two device types and three durations of use. Differences in frequency of use and perceived environmental barriers have been noted between power wheelchair and scooter users (25) that we anticipated would be captured by the WhOM. Gitlin et al. (26, 27) have found that AT outcomes vary according to the level of experience of the user. They consider the first six months as the initial use period while the period beyond the first year delineates expert use. The study design involved three groups (waiting for PMD provision, initial users and long-term users) to verify if the WhOM discriminated various durations of use.

Participants

The target population included individuals eligible for a power wheelchair funded by the Provincial Health Insurance Agency (Régie de l’assurance maladie du Québec [RAMQ]) or for a scooter from the Quebec Health and Social Services Ministry (Ministère de la santé et des services sociaux du Québec [MSSS]). The project was approved by the Institutional Review Boards of four rehabilitation centers. The rehabilitation centers provided lists of individuals who had received a PMD or were waiting to receive one. Names on the list were randomized on a per center basis and individuals were contacted following randomization. Those who were eligible were invited to participate to the study. Recruitment continued until the planned sample size was reached. Two samples of individuals were drawn from those who met the following criteria : i) eligible for PMD provision from RAMQ or MSSS, ii) age 50 and over, iii) first PMD provision, iv) ability to communicate by telephone in French (reliability sample), and in French or English (validity sample), v) duration of use was an additional inclusion criterion for the validity sample: group 1 was waiting for the device, group 2 was using the device for 1–6 months and group 3 was using the device for 12–18 months. Exclusion criteria were inability to use the PMD due to mechanical failure or hospitalization for more than 48 hours during the month preceding the interview.

The assessments took place between December 2006 and April 2007 for the reliability sample, and between June and October of summers 2007 and 2008 for the validity sample. Those timeframes insured comparable climatic conditions within each cohort.

Measurement tools

The WhOM requires the person to identify up to 5 wheelchair-related participation goals at home and 5 in the community. Part I rates the importance and satisfaction with participation for each goal on an eleven point scale (0 not important at all – 10 extremely important; 0 not satisfied at all – 10 extremely satisfied). Two mean scores and two total scores are computed: 1) Mean importance x satisfaction (MeanIMP*SAT): ranging from 0 to 100; 2) Mean satisfaction (MeanSAT): ranging from 0 to 10; 3) Total importance x satisfaction (TotIMP*SAT): ranging from 0 to 1000; and 4) Total satisfaction (TotSAT): ranging from 0 to 100. Higher scores are interpreted as better outcome. Part II includes three questions on body functions related to comfort, positioning and skin condition with an eleven-point scale (0 low score – 10 high score). The French version was developed based on a systematic methodology to insure that the original and adapted versions were equivalent (28–30). The translation/back-translation process, which involved bilingual PMD users and an expert committee is available from the author. A bilingual telephone script (French and English) was developed for the WhOM. It employed an alternative scale which used descriptive rather than numeric levels. The script also specified some cueing instructions that can be utilized to probe the answers of the participant over the telephone.

Satisfaction with the PMD was measured with the Quebec User Evaluation Satisfaction with Assistive Technology questionnaire (QUEST 2.0) (31), a 12-item questionnaire that generates 3 scores: an average satisfaction with “technology” score based on 8 items, an average satisfaction with “services” score derived from 4 items, and a “total” score averaged across the 12 items. Each item is scored using a 5-point satisfaction scale, with a score of 1 denoting ‘not satisfied at all’ and 5 indicating that the person is ‘very satisfied’.

The 10-item version of the Psychosocial Impact of Assistive Devices Scale (PIADS-10) (32) was used for self-rating perceptions of how assistive devices affect quality of life for aspects such as competence (e.g.feelings of independence), adaptability (e.g. willingness to try new things), and self-esteem (e.g. feelings of emotional wellbeing and happiness). Scores on each item can range from –3 (max. negative impact) to +3 (max. positive impact). The short version of the PIADS generates a total score, ranging between −3 and +3, that is averaged across 10 items.

The QUEST and PIADS-10 were chosen based on conceptual appropriateness, potential for telephone use (33) in both French and English and the quality of their measurement properties (31, 33–35).

Demographic and clinical background information including age, gender, diagnosis, type of PMD, living arrangement, geographic area, accessibility of residence and access to adapted transport were extracted from charts that contained the participant’s provincial wheelchair application forms.

Procedure for test-retest reliability and applicability

Telephone interviews were conducted with 40 participants by a rehabilitation clinician (examiner). The delay between the test (T1) and retest (T2) telephone interviews was 7 to 14 days as suggested by Streiner & Norman (23) and repeated by the same examiner. Both participants and examiners did not have access to the list of participation objectives nominated at T1 when the questionnaire was repeated. The initial interview was taped with a digital recorder and the duration of assessment was calculated with the mp3 file. The behavior coding technique of Fowler and Cannell (36) was used to systematically register behaviors that reflect the applicability of the questionnaire, such as the burden for the participant and the examiner (e.g. participants asking for clarifications, examiner reformulating a question, missing data, negative comments). To look at the stability of goals over time, each participation objective was coded with detailed four-level ICF codes.

Procedure for convergent and discriminant validity

The WhOM, QUEST and PIADS-10 were administered randomly to avoid order effects. All questionnaires were completed over one or two telephone sessions over a period of 7 days, except for two participants who needed three sessions over the same period.

Analyses

Descriptive statistics were computed and histograms were visually inspected for all variables. The proportion of scores at the top and bottom of the scales were calculated. A floor or ceiling effect was defined as a clustering of >20% of the responses at the minimal or maximal level of the scale based on recommendations by Andresen et al. (37). Assumptions for homoscedasticity were also verified with normality test results (Kolmogorov-Smirnov Z score; p< .05).

To evaluate the telephone applicability of the WhOM, the following behaviors were coded with the recorded interviews on a present/absent scale: respondent burden (asks clarifications, expresses negative comments verbally or non verbally, interrupts the examiner, doesn’t know the answer) and examiner burden (reformulates the question, simplifies the rating scale, uses probing, adds definitions). The percentage of occurrence (P_oc) of each behaviour was computed across all participants and regrouped at the item level. Based on Fowler and Cannell’s criteria (36) for behaviour coding, an item was considered problematic when the P_oc was above 15% (n>6).

The stability of the participation objectives between T1 and T2 was verified descriptively and statistically. The proportion who mentioned the same participation goals, based on 4-level ICF codes, and the proportions who changed goals were computed. Test-retest reliability was calculated with intraclass correlation coefficients from a two-way mixed effects model (ICC_2,1) with scores as a between-subjects random effect and assessment session as a within-subjects fixed effect (38). An ICC >.75 was considered high, between .75 and .40 was considered moderate, and <.40 was considered low (37). A reflect and square root transformation was applied to the MeanSAT score to correct a skewed distribution towards the higher end of the scale, as recommended by Tabachnik & Fidel (39).

Convergent validity was assessed using Spearman’s rho (r_S). Statistically significant correlations were interpreted as large when the coefficient was at least .50, moderate between .30 and .49, and small between .10 and .29 (40). Based on a previous validation study with the WhOM (20), it was hypothesized that the WhOM scores would be moderately and positively correlated with satisfaction with the AT (QUEST) and higher correlations were expected with psychosocial impacts of the PMD (PIADS-10). For discriminant validity testing, it was hypothesized that the importance of wheelchair participation goals and satisfaction would be higher for power wheelchair users compared to scooter users since power wheelchairs are usually provided to those with more severe motor impairments, and thus perform more activities with a PMD. Moreover, lower WhOM scores were expected for the group that was waiting for the device. Due to skewed distributions for some of the scores, nonparametric analyses were used to test the two hypothesis including the Krushkall-Wallis test (3 durations) and Mann-Whitney U test (2 device types) with a significance level of p<.05. To contrast the three durations, post-hoc tests were conducted with the Mann-Whitney U test, using a Bonferonni correction (α= .017).

Statistical analyses were performed with the Statistical Package for the Social Sciences (SPSS) version 16.0 (41).

Results

Descriptive results

The characteristics of two independent reliability and validity samples are presented in table I. In both samples, mean age was 65 years (SD=10) and a majority were women. The reliability sample was comprised of 40 persons who had been using their PMD for 2 to 15 months (mean=7.7 months, SD=3.1). Most of these participants lived in the community (87.5%; n=35), while some lived in long-term care facilities (10%; n=4). The validity sample comprised 116 persons either waiting for (n=42) or using (n=74) a PMD. This sample was similar to the reliability sample, except that the representation of rural areas was much lower (13.8%; n=16) and the proportion of users with access to adapted transportation was higher (69.8%; n=81).

Table I.

Characteristics of the two samples

Variable	Reliability sample (n=40)		Validity sample (n=116)
	n	%	n	%
Age (yr)
50–64	25	62.5	59	50.8
65–74	8	20.0	37	31.9
≥75	7	17.5	20	17.3
Gender
Male	17	42.5	47	40.5
Female	23	57.5	69	59.5
Main diagnosis
Neurological	21	52.5	58	50.0
Musculosqueletal	10	25.0	30	25.9
Medically complex	9	22.5	28	24.1
Type of PMD
Scooter	26	65.0	60	51.7
Power wheelchair	14	35.0	56	48.3
Living arrangement
Alone	15	43.0	32	27.6
Not alone	25	57.0	84	72.4
Geographic area
Urban	28	70.0	100	86.2
Rural	12	30.0	16	13.8
Accessibility of residence
Fully accessible	30	75.0	60	51.7
Partial or not accessible	10	25.0	56	48.3
Adapted transport
Privately owned vehicle	2	5.0	4	3.4
Public transportation	20	50.0	77	66.4
No adapted transport	18	45.0	35	30.2
Duration of PMD use
group 1: waiting	-	-	42	36.2
group 2: 1–6 months	-	-	35	30.2
group 3: 12–18 months	-	-	39	33.6

Test-retest reliability

The participation objectives mentioned at test and retest by the 40 participants are listed in table II by descending frequency for each ICF chapter. The proportion of community participation goals was three times higher than participation goals at home, with a marked predominance of recreation and leisure activities. Community goals predominated since a large proportion of participants had no participation goal at home (n=29; 73%) whereas only one did not mention any goal in the community (n=1; 2.5%). The proportion of participants who spontaneously mentioned the same participation objectives (based on the 4-level ICF code) or changed one of them was 90% (n=36) at home and 77.5% (n=31) in the community. The rest of the participants changed 2 objectives (2.5% [n=1] at home; 15.0% [n=6 community]) or 3 objectives (2.5% [n=1] at home; 7.5% [n=3] community) between T1 and T2. Descriptive data (mean and ranges) at T1 and T2, as well as reliability estimates are presented at table III. Computed test-retest reliability estimates were high for all WhOM scores (ICC=.77 to 1.00).

Table II.

Participation objectives at home and in the community coded with the ICF

ICF Code	ICF Chapter	Number of participation objectives		Examples of participation objectives
		At home	Community
9200	Recreation and leisure	12	56	Play bowling once a week all year round
4600	Moving around in different locations	6	13	Take daily rides to the park depending on climatic conditions
7500	Informal social relationships	2	9	Have a coffee with people at the shopping mall once a week
7600	Family relationships	0	8	Visit sister/brother/children once or twice a month depending on climatic conditions
6500	Caring for household objects	3	3	Shop once a week with partner all year round
9300	Religion and spirituality	1	3	Attend church activities once a week during spring, summer and fall
5700	Looking after one's health	1	2	Go to doctor's appointment when needed
8600	Economic transactions	0	3	Go to the bank without help once a week all year round
3600	Using communication devices	2	0	Get to the computer room to use internet twice per week, all year round
6300	Preparing meals	2	0	Cook all meals while using the PMD
8500	Employment (remunerative or not)	0	2	Use the PMD at work twice a week all year round
9100	Community life	0	2	Participate to the local fund-raising committee once a month
5500	Eating	1	0	Have lunch at the cafeteria everyday all year round (long-term care resident)

Note: Listed from most frequent to least frequent ICF codes

Legend: ICF= International Classification of Functioning, Disability and Health; 2-level codes are presented to aggregate results

Table III.

Test/retest reliability estimates of the WhOM scores (n=40 participants)

WhOM scores (theoretical range)	Test mean score (range)	Retest mean score (range)	ICC	ICC.95
Part I: Mean scores
MeanIMP*SAT (0–100)	79.8 (22.4–100)	77.5 (28–100)	.89	.78–.94
Mean SAT (0–10)	8.9 (4.8–10)	8.6 (4–10)	.79a	.59–.89
Part I: Total scores
TotIMP*SAT (0–1000)	312 (64–900)	293 (28–900)	.92	.84–.96
TotSAT (0–100)	35 (10–90)	33 (4–90)	.91	.83–.95
Part II: Body functions
Comfort (0–10)	8.7 (4–10)	8.8 (4–10)	.85	.71–.92
Position (0–10)	9.2 (6–10)	9.1 (4–10)	.77	.57–.88
Skin condition (0–10)	10 (0)	10 (0)	1.00	1.00–1.00

Legend: ICC= intra-class coefficient; ICC_.95= ICC 95% confidence interval;

^aICC calculated on transformed score (Reflect and square root transformation)

Applicability

The WhOM interview was administered in 3 to 25 minutes (mean=10.9, SD=5.2). The behaviour coding revealed that respondents could answer all items and that the questionnaire was well accepted since negative comments (n=1) and interruptions (n=2) were minimal. The total scores of the WhOM at T1 and T2 were well distributed, but the mean scores were negatively skewed (see table IV). A ceiling effect was identified for the MeanSAT and MeanIMP*SAT since more than 20% of the responses were clustered at the top of the scale. Statistically significant Kolmogorov-Smirnov test confirmed the distribution biases (p<.05). Regarding the distribution of body function scores, the observed range was restricted between 8 and 20 on a theoretical scale of 0 to 30, since none of the participants reported any skin breakdown.

Table IV.

Distribution of the WhOM scores at test and retest (n=40)

	Test (T1)				Retest (T2)
	Theoretical range	Actual range	% (n) bottom of scale	% (n) top of scale	Actual range	% (n) bottom of scale	% (n) top of scale
Part I: Mean scores
MeanIMP* SAT	0–100	22–100**	0 (0)	22.5 (9)	28–100**	0 (0)	15 (6)
MeanSAT	0–10	4.8–10*	0 (0)	27.5 (11)	4–10	0 (0)	25 (10)
Part I: Total scores
TotIMP* SAT	0–1000	64–900	0 (0)	0 (0)	28–900	0 (0)	0 (0)
TotSAT	0–100	10–90	0 (0)	0 (0)	4–90	0 (0)	0 (0)
Part II: Body functions (0–30)	0–30	11–20*	0 (0)	0 (0)	8–20**	0 (0)	0 (0)

Legend: Statistically significant Kolmogorov-Smirnov test:

^*p< 0.05;

^**p <0.001

Convergent and discriminant validity

Convergent validity estimates between the QUEST, PIADS-10 and WhOM are shown at table V. At a descriptive level, the 74 PMD users were generally ‘Quite satisfied (4/5)’ (66.2%; n=49) with their PMD as measured with the QUEST (mean=4.4/5; SD=.6; range=2.5–5.0). Positive psychosocial impacts were reported on average on the PIADS-10 (mean=1.9/3; SD=.8; range= −.2 to 3.0). The convergent validity analyses estimated moderate coefficients ranging from .36- .46 between all QUEST scores and mean WhOM scores (MeanIMP*SAT and MeanSAT). Correlations ranging from .31–.43 were moderate with the PIADS-10 for all WhOM scores except for TotSAT which was not significant. The strength of associations were slightly larger for the PIADS-10 when both importance and satisfaction were included in the WhOM scoring (MeanIMP*SAT and TotIMP*SAT) compared to corresponding satisfaction scores (MeanSAT and TotSAT). Total satisfaction (TotSAT) was not correlated significantly to any other variable and TotIMP*SAT had a small correlation with QUEST services.

Table V.

Convergent validity for initial and long-term users (n=74)

	WhOM Part I: Participation
	MeanIMP* SAT	Mean SAT	TotIMP* SAT	TotSAT
QUEST total score	.37**	.45**	.10	.03
QUEST technology	.41**	.46**	.10	.01
QUEST services	.36**	.42**	.24*	.19
PIADS-10	.43**	.33**	.31**	.21

Legend: 1= Spearman's rho;

^*p<0.05;

^**p<0.001

Discriminant validity compared participation outcomes for power wheelchair and scooter users for three durations of PMD use. Regarding duration of use, the Krushkall-Wallis test estimated statistically different WhOM scores across duration of use (p<.001) for all the WhOM scores. Significant contrasts were found between wait-list and both groups of users (Mann-Whitney U test, initial users p<.001; long term users p<.001) for all WhOM scores. There was no difference between initial and long term users (Mann-Whitney U test; p=.58–.83). Device type was a significant factor for one WhOM score (TotSAT; Mann-Whitney U test, p=.03), with significantly higher scores for power wheelchair users. Figure 1 shows separate analyses for power wheelchair and scooter users at home. A significant duration effect was found for power wheelchair users on satisfaction at home (MeanSAT Krushkall-Wallis, p<.05), but not for scooter users. In the community, a duration effect for power wheelchair users was significant for mean satisfaction (MeanSAT Krushkall-Wallis, p<.001, figure 2) and total satisfaction (TotSAT Krushkall-Wallis, p<.05). For scooter users, only satisfaction in the community (p<.001; see Mean SAT figure 2) differed according to duration of use.

Figure 1.

WhOM scores at home for two device types and three durations of use (N=116)

Figure 2.

WhOM scores in the community for two device types and three durations of use (N=116)

Discussion

This study examined the measurement properties of the telephone version of the WhOM with middle-aged and older PMD users with respect to reliability, applicability and validity. One of the strengths of the present study was the random selection of PMD users through institutions that held provincial or regional mandates for the provision of subsidized power wheelchairs and scooters. This design insured that participants representing various levels of participation and satisfaction were included. Another strength was the inclusion of users with different durations of use. Research on the impacts of PMDs in older adults is scarce and typically does not control for duration of use and device type (10).

All the two week test-retest reliability estimates were above the threshold of .75 suggested by Andresen (37). Moreover, a qualitative verification of the stability of the nominated objectives was performed and revealed that 90% spontaneously repeated the same participation objectives at home or modified one between the two assessments. This proportion was slightly lower (77.5%), meaning less stable, regarding the participation objectives in the community. In both environments, it must be noted that changes in the nominated objectives generally remained in the same ICF chapters at test and retest (e.g. different leisure activities within the recreation chapter). These results support the WhOM administration guidelines that suggest to remind the user of his initial participation objectives when performing the follow-up assessment. However, the assessment should not be deferred if the initial list of participation objectives is unavailable since the WhOM score remains stable.

We found that modifying the assessment format from face-to-face to telephone did not alter its applicability for middle-aged and older PMD users. The WhOM was well accepted and simple to administer in less than 15 minutes. It was well understood by middle-aged and older PMD users. The delimitation of the home versus community had to be clarified occasionally when the participation goals took place in the backyard or on the terrace of the residence. The telephone script comprised an alternative scale which used descriptive rather than numeric levels, which proved to be useful for some participants. The appraisal of the properties of a questionnaire must also consider the distribution of scores to rule out floor or ceiling effects. A clustering of responses at the maximal level of the scale was found for up to 27.5% of respondents on the MeanSAT score. Total scores might be a better alternative than mean scores to track change over time, especially if the same number of initial goals are rerated, since total scores displayed normal distributions and comparable reliability results to the original version tested by Garden et al. (ICC=.93; ICC_.95=.88–.96) (20).

As expected, our results support a moderate positive link with satisfaction with the device, as measured by the QUEST. However, the coefficients of .42 to .46 obtained with the present sample are slightly lower than estimated by the validation study of Garden et al. (20). That study found an association of .58 between mean WhOM and QUEST in younger long-term users of power and manual wheelchairs with spinal cord injury. Whether differences in eligibility criteria, diagnosis, age, duration of use, and types of devices explain that discrepancy between the samples remains a question.

A stronger positive correlation was expected with the PIADS-10 than with the QUEST, since the former is centered on the impacts on the user rather than on the characteristics of the device. The associations in our validity sample were slightly stronger, though moderate, with the PIADS-10 when both importance and satisfaction were considered. That would indicate a better psychological reaction to the assistive devices when users value the activities they are performing with it. The correlations in the present study were stronger than estimated by Buning et al. (42) who identified a small correlation of .21 between the PIADS and a generic occupational performance measure for PMD users.

The moderate associations between the WhOM, and both the QUEST and PIADS-10, suggest that each brings distinct contributions to the assessment of the outcomes of the PMD intervention. Higher levels of satisfaction with the device or positive psychosocial impacts do not necessarily imply that the participation objectives of the users are fully met and vice versa. This observation is in accordance with the theoretical models of the interaction between the user, the device and the activity in a specific context (43, 44). For example, the user might indicate that the device is technically adequate but still experience dissatisfaction when performing specific activities or life roles with the device.

Differentiating outcomes according to device types has been identified as a fundamental gap in the field of wheeled mobility (45). To our knowledge, very few tools can distinguish participation levels for various types of mobility-related assistive technology, besides the “Facilitators And Barriers Survey of environmental influences on participation among people with lower limb Mobility impairments” (FABS-M) (25). The present study showed that the WhOM captures participation differences between power wheelchair users and scooter users on their total satisfaction (TotSAT) with participation, possibly because the number of objectives identified by power wheelchair users was more elevated at home. Moreover, only power wheelchair use displayed a duration effect on satisfaction (MeanSAT, MeanIMP*SAT) with participation at home. The environment was a key aspect in differentiating outcomes. Participation in the community was most valued and appeared to be a key dimension for users of both device types, while participation at home was an aspect that was different for power wheelchair and scooter users. This has implications for the use of the WhOM. If the aim is to quantify participation at the individual level across time, total scores are indicated. On the other hand, mean scores are recommended for group comparisons because the scores are not affected by the number of goals.

There are limitations to this study. First, a cross-sectional design does not allow to state with confidence that the differences observed between the waiting group, and the initial and long-term users truly reflect differences due to duration of use. Future studies should use repeated measures across time and address the responsiveness of the WhOM. Second, although we attempted to control for climatic conditions by selecting a time-window limiting the overlay of summer and winter seasons, the true duration of experience six months post-delivery might differ for users who receive their device at the beginning of winter or in the middle of the summer. Third, subsidy guidelines were based on a medical prescription with specific eligibility criteria and the mandatory involvement of an occupational or physical therapist in the assessment and follow-up process. The generalization of the present results applies to populations with neurological, musculoskeletal or medically complex conditions requiring medically prescribed devices.

In conclusion, our results show that the WhOM is a stable, valid and applicable measure that is practical to use with a population of PMD users of age 50 and over. The results of this study are important because there is a need for wheeled device-specific tools that are applicable to older adults. The tool could discriminate users based on duration of use of PMDs and device type (power wheelchair vs scooter). Moreover, this client-centered measure is moderately linked to the perception of the client about his satisfaction with the device and its psychosocial impact, and therefore complements rather than replaces those measures.