The World Health Organization (WHO) defined health as a state of complete physical, mental, and social well-being 12. Ware proposed functional status, well-being, and general health perceptions as the minimum set of generic health concepts 13. Other models of health and HRQL have been proposed 14, but none has been generally accepted. There are significant differences in the domains included in the leading HRQL instruments; furthermore, domains with similar content may have different names in different instruments 151617.

The most comprehensive framework for describing health is the International Classification of Functioning, Disability and Health (ICF) 1819. The ICF considers four major areas of health and function, i.e., body structures (e.g., structure of lower extremity), body functions (e.g., movement functions), activities and participation (e.g., mobility), and environmental factors. For each area, there are multiple levels of classification. For example, mobility is divided into changing and maintaining body position, moving and handling objects, walking and moving, and moving around using transportation. Walking and moving, in turn, is subdivided into walking, moving around, moving around in different locations, and moving around using equipment. Finally, walking is classified into walking short distances, walking long distances, walking on different surfaces, and walking around obstacles. We felt that this multi-level structure and a large number of possible domains would make the ICF too complex for use as a measurement tool. Therefore, in developing our measurement system, we combined the ICF model with an empirical approach based on existing instruments.

Our objective was to create a large database of previously validated items that would serve as a starting point for the development of several item banks. To this end, we reviewed the content of a large number of published health and quality of life questionnaires, both generic and disease- or domain-specific. The review started with the instruments included in major texts and published literature reviews 20212223. These were supplemented with additional instruments known to the investigators. Literature searches were then performed to look for additional questionnaires. We first entered items from widely used multi-dimensional measures. The content of the database was evaluated continuously and items from other instruments were entered, selected primarily for their domain content and perceived status as standard or well established measures. After items from 32 instruments were entered (Table 1), a consensus was reached among the investigators that the database was sufficiently comprehensive for the purpose of our study.

The items were then reclassified using the ICF concepts of "body functions" and "activities and participation". Each item was described in terms of the source questionnaire, wording of the question and response options, original concept measured, and domain assigned according to our new classification. In this way, over 1,400 items were classified into the following 19 domains: lower extremity function, upper extremity function, pain, emotional function, cognitive function, communication, energy, sleep, vision, hearing, cardiopulmonary function, digestive function, sexual/reproductive function, urinary function, skin function/appearance, self-care activities, domestic activities, interpersonal activities, and major life activities.

Further work was limited to a smaller number of domains, as our main target population were persons with arthritis and related disorders. Based on the literature 24 and our experience in measuring health outcomes in musculoskeletal conditions we identified the following domains as being highly relevant to this population: self-care, domestic and major life activities, lower extremity function, upper extremity function, pain, and emotional function. The first 3 domains represented the ICF concept of participation 1819. Lower and upper extremity functions were conceptualized as ability to perform activities that depend on these functions, such as those in the ICF domains of walking and handling objects, respectively 19. We extended the concept of pain, which is part of sensory function in the ICF 19, to include discomfort, as this term has been used in some questionnaires 16. Finally, emotional function was conceptualized based on the ICF as the spectrum of feelings, such as joy, sorrow, anger, or anxiousness 19. We combined self-care, domestic and major life activities and modified the labels to arrive at the following five final domains: (1) Daily Activities, (2) Walking, (3) Handling Objects, (4) Pain or Discomfort, and (5) Feelings. Of the 1,400 items in the database, 624 were classified into these five domains and these items were considered for further testing and reduction.

To eliminate redundant items, i.e., items that measured the same facet of a given domain, the items were organized by content, grouping all similar items together. In this way it was possible to identify identical or very similar items and eliminate duplication. When choosing between items sharing similar content, we considered primarily the wording of the question and the format of the response options. While redundant items were removed, sometimes multiple items with similar content were included for empirical testing. This was particularly true in the Daily Activities and Pain or Discomfort domains, where the number of distinct areas of activity was limited. For example, items asking about the level of difficulty and degree of limitation due to health for the same type of activity were included in the final questionnaire. At this stage, the wording of most items and the number and wording of response options were modified to achieve a sufficient degree of uniformity. The level of functioning for items in the Daily Activities, Walking, and Handling Objects domains was measured in terms of difficulty, limitations, or need for help with specific activities. Pain or Discomfort was assessed in terms of impact, intensity, or frequency. Items measuring Feelings asked about the amount of time spent in a given emotional state. We included items assessing depression and anxiety, using both positive and negative wording. All items were worded to reflect a 4-week timeframe commonly used in HRQL instruments and had between 3 and 6 response options. We decided that a 4-week recall period was appropriate for studies in chronic conditions such as arthritis to remove the "noise" caused by short-term fluctuations in symptoms, although alternative versions of the questions with a shorter timeframe, e.g., 7 days or 24 hours, could be developed in the future.

The items were categorized according to the approximate level of HRQL they pertained to ("difficulty") to identify gaps and further redundancies. Upon careful inspection of the items in each domain, we noted that extreme levels of function were not covered well. Such items tend to have highly skewed response distributions and are often deleted from HRQL questionnaires in the content reduction phase. However, for an item bank it is important to include items that can discriminate at either a very high or very low level of function. The relative scarcity of such items, particularly those measuring the highest functional levels, required the development of new items. For example, in the walking domain, we included an item asking about difficulty running or jogging 20 miles to discriminate among relatively healthy, younger individuals. Similarly, in Handling Objects, we added items about carrying 100 and 200 lbs. An item about planning a suicide was included in Feelings to discriminate among severely depressed individuals. All new items used a standardized format, with a 4-week recall and 5 ordered response options.

The procedures described in the previous section reduced the number of items from over 600 to about 230. In the next step, the items in each domain were subjected to a multi-stage empirical pre-testing and iterative revision process 25. Twenty-four volunteers pre-tested the item pool. Subjects ranged from 25 to 86 years of age (mean = 46) and 71% were female. Most had completed at least some college or university. Nearly half reported having osteoarthritis or back pain. Some pre-tests were conducted in groups, others individually. Following completion of the questionnaire, a discussion was held about the clarity of instructions, format and wording, as well as reactions to item content, e.g., to identify items considered controversial or irrelevant. Content development continued through the pre-testing stage, with new items being developed from focus groups and individual interviews.

Most items identified by the subjects as unclear either referred to more than one concept (e.g., items combining activities such as eating and bathing) or were considered too lengthy. "Some questions have more than one idea, which makes it unclear how to answer." "Ask how difficult it would have been for you, not whether or not you have done the activity." "Some of the long questions are over-worded." Clarification was also sought on items referring to distances. "It's easier to think in terms of blocks than yards." Some participants reacted positively to the inclusion of multiple items addressing the same function, while others disliked the repetition. "It is well organized, and I liked the repetition." "The repetition was irritating and made it seem like a test." Difficulty in choosing the appropriate response because of recent health changes was also expressed. The items were revised or deleted as testing progressed, based on subjects' comments. The introduction to the questionnaire was modified to help clarify the purpose of the questionnaire and to help subjects decide how to respond if their health state had changed recently. All 11 subjects completing the last two versions found the instructions very clear and 9/11 found the meaning of the questions very clear.

In the final stage of content development, a nominal group technique was used, where members of the investigative team reviewed all the items and reached consensus on the final item pool. This process resulted in a 219-item item calibration questionnaire (ICQ). The questionnaire contained 43 items in the Daily Activities domain, 38 in Walking, 54 in Handling Objects, 39 in Pain or Discomfort, and 45 in Feelings.

Subjects in the item calibration study were patients drawn from two clinics at the Vancouver Hospital and Sciences Centre (VHSC) and a stratified random community sample in British Columbia (BC), Canada. We obtained a list of 554 patients with rheumatic conditions, treated by rheumatologists at the VHSC between 1994 and 2001. The vast majority had been diagnosed with rheumatoid arthritis (RA), although several patients had other types of inflammatory arthritis. We also obtained a list of 472 patients with radiographically confirmed osteoarthritis (OA) of the hip or knee waiting for joint replacement surgery. All patients able to complete the questionnaire were considered eligible for the study. For the community sample, a random computerized list of 3,000 telephone subscribers in BC, aged 18 years or older, was obtained to provide a representative sample of households in the province. The sample was randomly divided into two sub-samples. In 2,000 subscribers we asked that the questionnaire be completed by the adult in the household whose birthday date came next, following receipt of the questionnaire. In the remaining 1,000 subscribers we asked the oldest person in the household to complete the questionnaire. The reason for weighting the sample towards older persons was to increase the proportion of individuals with functional limitations.

A letter of introduction and the 219-item ICQ were mailed to each potential participant, along with a self-addressed pre-paid envelope. A reminder card was sent after one week. A second copy of the questionnaire was sent to non-respondents four weeks after the initial mailing, followed by a second reminder one week later. In the clinical samples, the remaining non-respondents were called to remind them to send in the questionnaire. Up to five phone calls were attempted at different times of the day and different days of the week and/or two voice messages were left. In the community samples, no phone calls were made to non-respondents; however, a cash draw incentive was offered to those completing the questionnaire. The study was approved by the University of British Columbia Ethics Board.

The items were analyzed in a step-wise fashion, as described in the literature 456. The steps in the analyses were as follows: 1) analysis of item dimensionality; 2) derivation of item parameters and option characteristic curves (OCCs); 3) analysis of item fit; and 4) analysis of differential item functioning. Extreme response categories with less than 5 responses were collapsed with the next most extreme category prior to the analysis to ensure statistical stability of item parameters.

For the purpose of item analysis we selected subjects with arthritis, as this was the main target population for our instrument. We received 331 questionnaires from patients in the rheumatology clinic, 340 from patients on the orthopedic waiting list, and 217 from respondents with RA or OA in the community sample. These 3 groups formed our analysis dataset (N = 888). The overall response rate among eligible subjects in the two clinical samples was 80%. The response rate in the community sample was 33%. Key characteristics of the respondents are presented in Table 2. The proportion aged 65 years or older differed significantly between the samples and ranged from 25% in the rheumatology clinic to 50% in the community sample. The majority of respondents were female in all three samples, but the proportion of females was highest among patients from the rheumatology clinic. About one third of the participants in all three samples had college/university education. Co-morbid conditions were fairly common across the three samples and their frequencies were likely influenced by the age and sex distributions. For example, 32 – 47% reported back pain, 9 – 15% reported heart disease, 5 – 8% reported diabetes, and 9 – 11% reported depression. The proportion reporting fair or poor self-rated health was high in patients from the rheumatology clinic (55%) and similar among those on the orthopedic waiting list (26%) and in the community sample (29%).

The results of factor analysis for each domain are presented in Table 3. The first factor explained between 58.3% (Feelings) and 72.3% (Walking) of the variance. All items loaded ≥0.4 on a single factor. RMS residual correlations were ≥0.1 for 4 items in the Daily Activities domain, 3 in the Walking domain, 11 in the Handling Objects domain, 7 in the Pain or Discomfort domain and 8 in the Feelings domain (Table 3). Most of these items had RMS residual correlations <0.12 and loadings >0.7 on a single factor; the largest RMS residual correlation was 0.15 (Item 16 in Pain or Discomfort). High RMS correlations were almost invariably associated with a highly skewed response distribution. The scree plots suggested a single factor for all domains, although we noted a slight indication of possible additional factors in the Handling Objects and Feelings domains (data not shown). To explore this further, we performed several additional factor analyses allowing for more than 1 factor and reviewed the content of the items loading on different factors to determine if these factors could represent distinct conceptual facets of a given domain. We also applied graphical methods of analysis, whereby the items were ordered by location and displayed as a series of lines, using different colors for items loading on different factors. These graphs showed very little overlap between the factors (data not shown). Both graphical analysis and content review indicated that these potential factors were related to item difficulty rather than content. When we considered all the results of factor analyses, the five domains were deemed sufficiently unidimensional for IRT modeling and no items were dropped at this stage.

Items that showed statistically significant and substantial DIF for age and/or sex are listed in Table 4. For example, Items 40, 41 and 42 in the Daily Activities domain, all pertaining to traveling, showed DIF for age, whereas Item 37 in the Handling Objects domain (putting hand in a pocket) showed DIF for sex. Item 31 in Feelings, asking about the occurrence of crying spells, also had DIF with respect to sex. Three anxiety-related items in this domain displayed DIF for age (feeling calm and peaceful, worrying about the future, and feeling carefree). All items displaying significant and substantial DIF were dropped after we assessed their statistical properties and their contribution to the content of their respective domains.

The numbers of items that were retained/eliminated in each of the five domains were as follows: Daily Activities 39/4, Walking 31/7, Handling Objects 45/9, Pain or Discomfort 36/3, and Feelings 37/8. Thus, the total number of items in the final domains was 188. Unadjusted p-values from the item fit chi-square test for these items are presented in Table 5, with lower p-values indicating worse item fit. Unadjusted p-values ≤ 0.01 were observed for 3 items in Daily Activities, 0 in Walking and 1 in each of the remaining domains. While many items displayed statistically significant DIF, the magnitude of DIF, as measured by change in the Nagelkerke maximal rescaled R-square, was generally small (data not shown).

Descriptive statistics for the distribution of item parameters are shown in Table 6. The range of the location parameter was large compared to a standard normal distribution in all five domains, indicating that the items covered a wide range of the construct measured. The Daily Activities domain had the narrowest range (-2.24 to 0.55) and the Handling Objects domain had the widest range (-1.70 to 2.27). Items in the Walking domain tended to have the highest slopes, although most items in all five domains had slopes greater than 1.0. The mean (median) slope ranged from 1.15 (1.07) in Feelings to 1.73 (1.75) in Walking (Table 6) (see also the Appendix [Additional file 1]).

Examples of OCCs are given in Figure 2. For each domain we show 2 items that differ in location, slope or both. For example, the OCCs for Item 23 in Handling Objects (difficulty brushing teeth) are shifted to the left, with the probability of selecting option 1 (no difficulty) reaching almost 100% for estimated trait scores greater than 0. The OCCs are fairly steep, consistent with a relatively high slope for this item. The item provides information for lower levels of health in the Handling Objects domain. By contrast, Item 50 (difficulty lifting and moving heavy furniture) is more informative at the higher end of the trait spectrum where item 23 is virtually non-informative. The OCCs for this item are less steep and the slope is lower.

Finally, overall test (domain) information functions, describing the amount of psychometric information for each domain according to trait level, are shown in Figure 3. As one would expect, the curves are mound-shaped, indicating that information is not evenly distributed. Also, the curves are shifted slightly to the left, towards lower levels of health, especially for Handling Objects. Nevertheless, these curves show that information is available for a wide range of functional levels in each domain. Since information is related to discrimination, the domain-specific scores should be able to discriminate between different levels of HRQL among relatively healthy people as well as among those with severe health problems.