Patients and setting

Data was collected in the Department of Physical Medicine and Rehabilitation at the Medical Faculty of Ankara University, Turkey. A total of 100 outpatients diagnosed as knee and/or hip OA according to the American College of Rheumatology criteria for the classification and reporting OA of knee and hip were included in the study [9, 10]. Patients with concomitant uncontrolled or severe systemic diseases, any recent surgery that might affect their health status, and any cognitive impairment that would preclude participation in the study were excluded. The study was approved by the Ethical Committee of the Faculty of Medicine, Ankara University. All patients gave informed consent and the study was carried out in compliance with Helsinki Declaration.

Assessment

The assessment included the administration of the ICF Comprehensive Core Set for OA, the Western Ontario and McMaster Universities Index of Osteoarthritis (WOMAC, V3.1) [11] and the Short Form-36 Health Survey v1.0 (SF-36^®) [12]. The scoring of ICF Core Set for all patients was performed by the physical and rehabilitation medicine specialists who were trained in a structured one-day workshop organized by the researchers of the WHO ICF Collaborating Center at the Ludwig-Maximilian University in Munich. These specialists took part in the International Validation Studies of Core Sets and were experienced in the scoring system since they collected the data of many patients with various musculoskeletal conditions such as osteoarthritis, low back pain, rheumatoid arthritis and chronic widespread pain. The questionnaires WOMAC and SF-36 were either self-completed by patients or the assessors administered them to those who were illiterate. Sociodemographic (age, gender, educational level, employment status) and clinical data (disease duration, location, comorbidities) were also recorded.

The ICF Comprehensive Core Set for OA consists of 13 categories from the component Body functions (BF), 6 from the component Body Structures (BS), 19 from the component Activities and Participation (AP), and 17 from the component Environmental Factors (EF) [6]. A generic qualifier scale was used to evaluate the extent of a patient's problem in each of the ICF categories. The qualifier scale of the components BF, BS and AP have five response levels, ranging from 0 to 4: no/mild/moderate/severe/complete problem. The qualifier scale of the component EF has 9 response levels, ranging from -4 to +4. A specific environmental factor can be a barrier (-1 to -4), or a facilitator (1 to 4), or can have no influence (0) on the patient's life. If a factor has an influence, the extent of the influence (either positive or negative) can be coded as mild, moderate, severe, or complete. In addition, there are two other response options "8 (not specified)" and "9 (not applicable)" for all ICF categories.

The WOMAC is a disease-specific index developed for OA of the knee or hip [11]. It consists of 24 items in three subscales: pain (5 items), stiffness (2 items), and physical function (17 items). There are five response options for every question ('0' none, '1' mild, '2' moderate, '3' severe and '4' extreme) in Likert form. In this study, validated Turkish version of WOMAC [13] was used and the scores were presented as 0-10 for each WOMAC subscale after a normalization procedure [11, 14]. The summation of equally weighted three subscales provided a single value for WOMAC total score, thus being 0-30.

Health-related quality of life (HRQoL) was evaluated using the SF-36 questionnaire [15]. It contains 36 items that measure perceived health in 8 scales, namely, physical functioning (PF), role-physical (RP), bodily pain (BP), general health (GH), vitality (V), social functioning (SF), role-emotional (RE), and mental health (MH), with higher scores (range 0-100) reflecting better perceived health. Additionally, two summary scores can be obtained; the Physical Component Summary (PCS) score and the Mental Component Summary (MCS) score. The Turkish version of the SF-36 was used in the study [16].

Internal Construct Validity

The internal construct validity of the items of the ICF Core Set for OA, proposed as a scale for each ICF component, was tested by Rasch analysis. This is the formal testing of an assessment or a scale against a mathematical measurement model which defines how interval scale measurement can be derived from ordinal questionnaires [17–19]. This model assumes that the probability of a given respondent affirming an item is a logistic function of the relative distance between the item difficulty and the person ability on a linear scale. Thus, for example, in the case of mobility, the probability of a person affirming a (dichotomous) item about mobility is a logistic function of the relative distance between the level of mobility expressed by the item (the item difficulty), and the level of mobility of the person (the person ability). The model estimates person ability independent of the distribution of the population, and item difficulty independent of the person ability [20]. Master's partial credit model (PCM) which is an extension of the Rasch dichotomous model for polytomous (more than two response categories) items was used in this study [21].

The process of Rasch analysis is iterative, certain pathways are applied to each scale where an item set is intended to be summated to give a score. Initially, where polytomous items are involved, the response categories are examined for correct ordering. This is reflected by successive thresholds (point at which probability of being in adjacent thresholds is equal) demonstrating increasing levels of the construct being measured. The respondents' inconsistent use of response options can result in disordered thresholds and usually, in these circumstances, the collapsing of categories improves overall fit to the model [22].

Following this a range of tests are undertaken with respect to local dependency, probabilistic ordering (fit), unidimensionality and differential item functioning (DIF). The assumption of local independence implies that when the 'Rasch factor' has been extracted, that is, the main scale, there should be no leftover patterns in the residuals [23]. When a pair of items has a residual correlation of 0.20 or more than the average residual correlation, this is indicative of local response dependency between the items [24]. Such dependency inflates reliability as the items are, in practice, near replications of each other. This issue is dealt with by creating testlets - summary scores from the items that are locally dependent, which are then treated as one new larger variable [25]. Testlets were created considering the contents (what they assess) and response dependency of the items where mostly clinically relevant items were found to be locally dependent.

A variety of fit statistics are used to test if the data conform to Rasch model expectations. In the RUMM2030 programme [26], two are item-person interaction statistics transformed to approximate a z score, representing a standardized normal distribution. If the items and persons fit the model, these interaction statistics would have a mean of approximately zero and a standard deviation (SD) of one. A third summary statistic is a summed chi-square within groups defined by their position on the trait, where the overall chi-square for items is summed to give the item trait interaction statistic, testing the property of invariance across the trait. A significant chi-square indicates that the hierarchical ordering of the items varies across the trait, so compromising the required property of invariance. The significance of all chi-square fit statistics are Bonferroni adjusted to account for multiple testing [27]. In addition to these overall summary fit statistics, individual person- and item-fit statistics are presented, as (a) residuals (a summation of individual person and item deviations), (b) as a chi-square statistic, and (c) as an analysis of variance (ANOVA) with the residuals summed across the main effects of class intervals. Fit residuals between ± 2.5 are deemed to be adequate. These are summated within ability groups to provide the basis of the ANOVA analysis.

A formal test of the assumption of unidimensionality is undertaken by performing a principle component analysis (PCA) of the residuals. Items with the highest positive and negative correlations on the first residual factor are used to construct two smaller scales, anchored to the item difficulties of the main analysis [28]. The person estimates derived from these two subsets of items are then contrasted for each individual by a t test. A significant difference would be expected to occur by chance in 5% of the cases. Consequently, the percentage of tests outside the range ± 1.96 is reported, together with a 95% binomial confidence interval. This interval should overlap 5% for a non-significant finding to confirm unidimensionality.

Items are also tested for DIF. In the framework of Rasch measurement, the scale should be free of item bias or DIF [29]. DIF occurs when different groups within the sample (e.g., males and females), despite equal levels of the underlying characteristic being measured, respond in a different manner to an individual item. For example, men and women with equal levels of mobility may respond systematically differently to a mobility item such as walking 100 metres unaided. DIF can be detected both statistically and graphically. In the current analysis, DIF was tested by age, gender, years of education, and disease duration. The statistical test for DIF is an ANOVA, with main effects, for example for gender, and ability level. This examines the main effect for gender (uniform DIF) where any difference is constant across the trait. An interaction effect between ability level and the contextual factor under investigation (e.g. gender) identifies non-uniform DIF, where the difference between groups varies across the trait.

For item sets which constitute a potential new scale, all the above Rasch assumptions are considered together to determine which items are most suitable for retention. Poor items are removed, and the data refitted to the model until an adequate locally independent, unidimensional scale, free of DIF, is achieved. Finally the targeting and Person Separation Index (PSI) reliability of the scale are considered. A scale is perfectly targeted when the mean of the persons is the same as the mean of the items on their shared common metric. PSI is an estimate of internal consistency reliability and can be interpreted much the same as Cronbach's alpha, but has the linear transformation from the Rasch model substituted for the ordinal raw score [30].

Reliability

Reliabilities of ICF components or proposed scales were initially tested by internal consistency which is an estimate of the degree to which its constituent items are interrelated, and is assessed by Cronbach's alpha coefficient [31]. Usually a reliability of 0.70 is required for analysis at the group level, and values of 0.85 and higher for individual use [32]. Subsequently reliability was further tested by the PSI from the Rasch analysis. Where the distribution is normal these two reliability indicators are equivalent, but where distributions are skewed, the PSI gives a more accurate indication of internal consistency reliability.

External construct validity

External construct validity was determined by testing for expected associations of ICF components or proposed scales with WOMAC and SF-36 through the process of convergent construct validity [33]. In this study, the degree of associations was analyzed by Spearman's correlation coefficient.

Sample size and statistical software

For the Rasch analysis, a sample size of 100 patients will estimate item difficulty, with α of 0.05, to within ± 0.5 logits [34]. Bonferroni correction was applied to both fit and DIF statistics due to the multiple testing [27]. Statistical analysis was undertaken with SPSS 11.5, Rasch analysis with RUMM2030 package [26].

Patient characteristics

Internal construct validity

"Body functions and body structures" (BF-BS) component

Initially we analysed the BF and BS items separately, but there was a problem with the 'sensation of pain' item (b280) which would have to be deleted. From a clinical point of view this was unacceptable, and so the potential of merging the two domains together was examined. Here we discovered that there was also local dependency across the 'b280 sensation of pain' and 's750 structure of the lower extremity' items. This suggested that in this group of people with hip and knee osteoarthritis, there may be some overlap between categories across domains. Consequently we merged the BF and BS items.

Starting with 19 items, 10 "body functions" and 3 "body structures" categories displayed disordered thresholds, necessitating collapsing of response options. Following this, four testlets were created in order to overcome the problem of local dependency (testlet1: b280, s750; testlet2: s720, s730; testlet3: b715, b720, b760; testlet4: b730, b740, s740). After this modification, fourth testlet was removed due to the lack of fit. The remaining 16 items were found to fit the model (given a Bonferroni adjustment fit level of 0.003) (Table 2), with an overall mean item fit residual of 0.138 (SD 0.921) and mean person fit residual of -0.147 (SD 0.652). Item-trait interaction was non-significant, supporting the invariance of items (chi-square 40.83 (df = 24), p = 0.017). The PSI was good (0.76) indicating the ability of the scale to differentiate between 3 groups of patients. All items were free of DIF by age, gender, years of education and disease duration. Finally, using the PCA of residuals and obtaining two further estimates, no significant difference in person estimates (t = 9.0%; CI 4.7%-13.3%) was found between the two subsets, thus supporting the unidimensionality of the scale. Although mean person location of -2.146 was less than that of the items, indicating an offset of persons (i.e. less impairment) to the centre of the scale, the distribution of thresholds was wide enough to maintain the ability to statistically discriminate the respondents (Figure 1).

ICF code	ICF category title	Location	SE	Individual Item Fit Residual	Chi-Square Test Statistics	p
Testlet1
b280 s750	Sensation of pain Structure of lower extremity	-1.682	0.075	2.286	8.795	0.012
Testlet2
s720 s730	Structure of shoulder region Structure of upper extremity	-0.723	0.098	0.735	3.481	0.175
Testlet3
b715 b720 b760	Stability of joint functions Mobility of bone functions Control of voluntary movement functions	-0.408	0.109	0.463	1.208	0.547
b130	Energy and drive functions	-0.383	0.117	-0.611	5.219	0.074
b134	Sleep functions	-1.096	0.147	-0.673	3.947	0.139
b152	Emotional functions	-0.943	0.116	-0.207	1.773	0.412
b710	Mobility of joint functions	-1.274	0.125	-0.714	3.305	0.192
b735	Muscle tone functions	3.319	0.776	-0.710	1.882	0.390
b770	Gait pattern functions	0.030	0.205	0.224	7.326	0.026
b780	Sensations related to muscles and movement functions	0.370	0.162	0.083	0.153	0.926
s770	Additional musculoskeletal structures related to movement	0.425	0.190	1.207	0.112	0.946
s799	Structures related to movement, unspecified	2.365	0.346	-0.432	3.634	0.163

"Activities" (A) component

Starting with 11 items, five items displayed disordered thresholds, necessitating collapsing of categories. After creating two testlets (testlet1: d430, d440, d445; testlet2: d410, d415, d455) and removing testlet2 due to misfit to the model, an eight-item scale satisfied Rasch assumptions (given a Bonferroni adjustment fit level of 0.008) (Table 3), with an overall mean item fit residual of 0.216 (SD 1.237) and mean person fit residual of -0.241 (SD 0.894). Item-trait interaction was non-significant, supporting the invariance of items (chi-square15.67 (df = 12), p = 0.207). The PSI was good (0.86) indicating the ability of the scale to differentiate between 4 groups of patients. No significant difference was found in person estimates (t = 7.6%; CI 3.2%-12.1%) between two subsets, thus supporting the unidimensionality of the scale. All items were free of DIF. The mean person location of -1.678 was less than the average of the items indicating that the patients were more active than the average level of activity of the scale (Figure 2).

ICF code	ICF category title	Location	SE	Individual Item Fit Residual	Chi-Square Test Statistics	p
Testlet1
d430 d440 d445	Lifting and carrying objects Fine hand use Hand and arm use	0.156	0.087	-0.235	0.471	0.790
d450	Walking	-0.799	0.143	2.069	5.327	0.070
d510	Washing oneself	-0.261	0.150	-0.831	4.122	0.127
d530	Toileting	1.216	0.167	1.482	0.907	0.635
d540	Dressing	0.667	0.221	-0.622	3.326	0.190
d640	Doing housework	-0.978	0.138	-0.566	1.519	0.468

"Participation" (P) component

Starting with eight items, eight items displayed disordered thresholds, necessitating collapsing of categories. After creating one testlet from the items "d470 and d475" and removing d850 due to DIF by gender, the remaining seven items were found to fit the model (given a Bonferroni adjustment fit level of 0.008) (Table 4), with an overall mean item fit residual of 0.759 (SD 0.986) and mean person fit residual of -0.310 (SD 1.187). Item-trait interaction was non-significant, supporting the invariance of items (chi-square 15.88 (df = 12), p = 0.197). The PSI was good (0.87) indicating the ability of the scale to differentiate between 4 groups of patients. Finally, using the PCA of residuals and obtaining two further estimates, no significant difference in person estimates (t = 7.6%; CI 3.2%-12.1%) was found between the two subsets, thus supporting the unidimensionality of the scale. All items were free of DIF. The mean person location of -1.143 indicated that the patients were participating at a higher level than the average of the scale (Figure 3).

ICF code	ICF category title	Location	SE	Individual Item Fit Residual	Chi-Square Test Statistics	p
Testlet1
d470 d475	Using transportation Driving	-0.288	0.283	2.001	7.227	0.027
d620	Acquisition of goods and services	-0.386	0.166	1.064	0.783	0.676
d660	Assisting others	-0.139	0.134	-0.807	4.221	0.121
d770	Intimate relationships	1.161	0.241	1.104	0.407	0.816
d910	Community life	-0.480	0.137	1.146	0.954	0.621
d920	Recreation and leisure	0.132	0.205	0.048	2.283	0.319

"Environmental Factors" (EF) component

While the original qualifier scale of the environmental factors ranged from -4 to +4, these 9 response levels do not represent a cumulative measurement of the impact of those environmental factors. As a result of this, barriers were rescored to 0, no influence was rescored to 1 and facilitators to 2.

Starting with the 17 items in the core set, 9 items displayed disordered thresholds, necessitating collapsing of response options. Following this, three testlets were created in order to overcome the problem of response dependency (testlet1: e120, e135, e150; testlet2: e310, e355, e410, e450, e540, e580; testlet3: e110, e225, e340, e460). After removal of the third testlet due to the lack of fit, fit to the model for the remaining thirteen-item scale was now satisfactory (given a Bonferroni adjustment fit level of 0.008) (Table 5). Overall mean item fit residual was -0.079 (SD 2.200) and mean person fit residual was -0.491 (SD 1.173). Item-trait interaction was non-significant, supporting the invariance of items (chi-square 15.73 (df = 12), p = 0.204). The PSI was good (0.71) indicating the ability of the scale to differentiate between 2 groups of patients. The unidimensionality of the scale was supported by the individual t-test showing 4.0% of tests as significant (CI -0.3%-8.3%). Given the mean person location was 2.222, this indicated that the majority of environmental factors experienced by the patients were facilitators (Figure 4). All items were free of DIF.

ICF code	ICF category title	Location	SE	Individual Item Fit Residual	Chi-Square Test Statistics	p
Testlet1
e120 e135 e150	Products and technology for personal indoor and outdoor mobility and transportation Products and technology for employment Design, construction and building products and technology of buildings for public use	1.316	0.136	0.888	1.156	0.561
Testlet2
e310 e355 e410 e450 e540 e580	Immediate family Health professionals Individual attitudes of immediate family members Individual attitudes of health professionals Transportation services, systems and policies Health services, systems and policies	1.113	0.048	-3.935	3.269	0.195
e115	Products and technology for personal use in daily living	-0.112	0.106	-1.141	3.654	0.161
e155	Design, construction and building products and technology of buildings for private use	1.901	0.128	2.397	4.402	0.111
e320	Friends	-2.579	0.109	0.520	0.560	0.756
e575	General social support services, systems and policies	-1.639	0.130	0.796	2.688	0.261

Reliability

Reliabilities of BF-BS, A, P, and EF were good with Cronbach's alpha values of 0.79, 0.86, 0.88, and 0.83 and, PSI's of 0.76, 0.86, 0.87 and 0.71, respectively.

External construct validity