An introduction to the Rasch measurement model: an example using the Hospital Anxiety and Depression Scale (HADS)

Measuring the impact and distress of osteoarthritis from the patients' perspective

Background

To assess the internal construct validity of the Perceived Impact of Problem Profile (PIPP), a patient based outcome measure based on the International Classification of Functioning, Disability and Health (ICF), which assesses impact and distress, in an osteoarthritis (OA) cohort.

Methods

A questionnaire comprising the 23-item PIPP, which assesses five domains (mobility, participation, self care, psychological well being and relationships), the Western Ontario McMasters University Osteoarthritis Index (WOMAC), the General Well-Being Index (GWBI), and the Hospital Anxiety and Depression Scale (HADS) was posted to people with clinician diagnosed OA. Assessment of the internal construct validity of the PIPP was undertaken using Rasch analysis performed with RUMM2020 software and concurrent validity through comparator measures.

Results

Two hundred and fifty-nine participants with OA responded. Analysis of the five individual domains of the PIPP indicated that there was good fit to the Rasch model, with high person separation reliability. One item required removal from the Mobility subscale and the Participation subscale. There were strong correlations between the PIPP Mobility scores and the WOMAC disability and pain subscales (rho = .73 and rho = .68), and between the PIPP Psychological well-being and HADS Depression (rho = .71) and GWBI (rho = -.69). High inter-correlations between the impact and distress subscales for each domain (range rho = .85 to .96), suggested redundancy of the latter.

Conclusion

This study demonstrates that the PIPP has good psychometric properties in an OA population. The PIPP, using just the impact subscales, provides a brief, reliable and valid means of assessing the impact of OA from the individual's perspective and operationalizing the bio-psychosocial model by the application of a single multi-domain questionnaire.

A review of instruments assessing participation in persons with spinal cord injury

STUDY DESIGN

A systematic search of the literature.

OBJECTIVES

To critically review instruments that assess participation in persons with spinal cord injury (SCI).

SETTING

Vancouver, British Columbia.

METHODS

Four electronic databases (MEDLINE/PubMed, CINAHL, EMBASE and PsychInfo) were searched for studies published between 1980 and March 2008. Instruments were included if information was published in English in at least one peer-reviewed journal on its measurement properties (reliability, validity and responsiveness) in a sample that included adults with SCI. Instruments were evaluated using criteria proposed for disability outcome measures.

RESULTS

Six instruments were included: Craig Handicap Assessment and Reporting Technique (CHART); Impact on Participation and Autonomy Questionnaire (IPA); Assessment of Life Habits Scale (Life-H); Occupational Performance History Interview; Physical Activity Recall Assessment for People with Spinal Cord Injury; and Reintegration to Normal Living Index. Evidence supporting the reliability of the instruments was reported for four of the six instruments and was adequate. Validity was assessed in all the instruments. Only the Life-H and CHART have been compared with each other. No evidence on responsiveness was available.

CONCLUSION

The instruments differ in how participation is operationalized. Currently, the CHART that measures objective aspects of participation has the most evidence supporting its measurement properties. More evidence is becoming available for instruments such as the IPA, which consider the person's perspective. It is important to determine what information about participation is required before selecting an instrument.

Internal construct validity of the Warwick-Edinburgh Mental Well-being Scale (WEMWBS): a Rasch analysis using data from the Scottish Health Education Population Survey

Background

The Warwick-Edinburgh Mental Well-Being Scale (WEMWBS) was developed to meet demand for instruments to measure mental well-being. It comprises 14 positively phrased Likert-style items and fulfils classic criteria for scale development. We report here the internal construct validity of WEMWBS from the perspective of the Rasch measurement model.

Methods

The model was applied to data collected from 779 respondents in Wave 12 (Autumn 2006) of the Scottish Health Education Population Survey. Respondents were aged 16–74 (average 41.9) yrs.

Results

Initial fit to model expectations was poor. The items 'I've been feeling good about myself', 'I've been interested in new things' and 'I've been feeling cheerful' all showed significant misfit to model expectations, and were deleted. This led to a marginal improvement in fit to the model. After further analysis, more items were deleted and a strict unidimensional seven item scale (the Short Warwick Edinburgh Mental Well-Being Scale (SWEMWBS)) was resolved. Many items deleted because of misfit with model expectations showed considerable bias for gender. Two retained items also demonstrated bias for gender but, at the scale level, cancelled out. One further retained item 'I've been feeling optimistic about the future' showed bias for age. The correlation between the 14 item and 7 item versions was 0.954.

Given fit to the Rasch model, and strict unidimensionality, SWEMWBS provides an interval scale estimate of mental well-being.

Conclusion

A short 7 item version of WEMWBS was found to satisfy the strict unidimensionality expectations of the Rasch model, and be largely free of bias. This scale, SWEMWBS, provides a raw score-interval scale transformation for use in parametric procedures. In terms of face validity, SWEMWBS presents a more restricted view of mental well-being than the 14 item WEMWBS, with most items representing aspects of psychological and eudemonic well-being, and few covering hedonic well-being or affect. However, robust measurement properties combined with brevity make SWEMWBS preferable to WEMWBS at present for monitoring mental well-being in populations. Where face validity is an issue there remain arguments for continuing to collect data on the full 14 item WEMWBS.

Variability in depression prevalence in early rheumatoid arthritis: a comparison of the CES-D and HAD-D Scales

Background

Depression is common in rheumatoid arthritis (RA), however reported prevalence varies considerably. Two frequently used instruments to identify depression are the Center for Epidemiological Studies Depression (CES-D) scale, and the Hospital Anxiety and Depression Scale (HADS). The objectives of this study were to test if the CES-D and HADS-D (a) satisfy current modern psychometric standards for unidimensional measurement in an early RA sample; (b) measure the same construct (i.e. depression); and (c) identify similar levels of depression.

Methods

Data from the two scales completed by patients with early RA were fitted to the Rasch measurement model to show that (a) each scale satisfies the criteria of fit to the model, including strict unidimensionality; (b) that the scales can be co-calibrated onto a single underlying continuum of depression and to (c) examine the location of the cut points on the underlying continuum as indication of the prevalence of depression.

Results

Ninety-two patients with early RA (62% female; mean age = 56.3, SD = 13.7) gave 141 sets of paired CES-D and HAD-D data. Fit of the data from the CES-D was found to be poor, and the scale had to be reduced to 13 items to satisfy Rasch measurement criteria whereas the HADS-D met model expectations from the outset. The 20 items combined (CES-D13 and HADS-D) satisfied Rasch model expectations. The CES-D gave a much higher prevalence of depression than the HADS-D.

Conclusion

The CES-D in its present form is unsuitable for use in patients with early RA, and needs to be reduced to a 13-item scale. The HADS-D is valid for early RA and the two scales measure the same underlying construct but their cut points lead to different estimates of the level of depression. Revised cut points on the CES-D13 provide comparative prevalence rates.

An initial application of computerized adaptive testing (CAT) for measuring disability in patients with low back pain

BACKGROUND

Recent approaches to outcome measurement involving Computerized Adaptive Testing (CAT) offer an approach for measuring disability in low back pain (LBP) in a way that can reduce the burden upon patient and professional. The aim of this study was to explore the potential of CAT in LBP for measuring disability as defined in the International Classification of Functioning, Disability and Health (ICF) which includes impairments, activity limitation, and participation restriction.

METHODS

266 patients with low back pain answered questions from a range of widely used questionnaires. An exploratory factor analysis (EFA) was used to identify disability dimensions which were then subjected to Rasch analysis. Reliability was tested by internal consistency and person separation index (PSI). Discriminant validity of disability levels were evaluated by Spearman correlation coefficient (r), intraclass correlation coefficient [ICC(2,1)] and the Bland-Altman approach. A CAT was developed for each dimension, and the results checked against simulated and real applications from a further 133 patients.

RESULTS

Factor analytic techniques identified two dimensions named "body functions" and "activity-participation". After deletion of some items for failure to fit the Rasch model, the remaining items were mostly free of Differential Item Functioning (DIF) for age and gender. Reliability exceeded 0.90 for both dimensions. The disability levels generated using all items and those obtained from the real CAT application were highly correlated (i.e. > 0.97 for both dimensions). On average, 19 and 14 items were needed to estimate the precise disability levels using the initial CAT for the first and second dimension. However, a marginal increase in the standard error of the estimate across successive iterations substantially reduced the number of items required to make an estimate.

CONCLUSION

Using a combination approach of EFA and Rasch analysis this study has shown that it is possible to calibrate items onto a single metric in a way that can be used to provide the basis of a CAT application. Thus there is an opportunity to obtain a wide variety of information to evaluate the biopsychosocial model in its more complex forms, without necessarily increasing the burden of information collection for patients.

Brazilian WHOQOL-OLD Module version: a Rasch analysis of a new instrument

OBJECTIVE

To evaluate the Brazilian version of WHOQOL-OLD Module and to test potential changes to the instrument to increase its psychometric adequacy.

METHODS

A total of 424 older adults living in a city in Southern Brazil completed the WHOQOL-OLD instrument, in 2005. Rasch analysis was used to explore the psychometric performance of the scale, as implemented by the RUMM2020 software. Item-trait interaction, threshold disorders, presence of differential item functioning and item fit, were analyzed.

RESULTS

Two ("death and dying" and "sensory abilities") out of six domains showed inadequate item-trait interactions. Rescoring the response scale and deleting the most misperforming items led to scale improvement. The evaluation of domains and items individually showed that the "intimacy" domain does perform well in contrast to the findings using the classical approach. In addition, the "sensory abilities" domain does not derive an interval measure in its current format.

CONCLUSIONS

Unidimensionality and local independence were seen in all domains. Changes in the response scale and deletion of problematic items improved the scale's performance.

Construct validity of a 12-item WOMAC for assessment of femoro-acetabular impingement and osteoarthritis of the hip

OBJECTIVE

Evaluation of the internal construct validity of the Western Ontario and McMaster Universities (WOMAC) Osteoarthritis Index adapted for use in patients with femoro-acetabular impingement (FAI) and osteoarthritis (OA) of the hip.

METHODS

Distribution of a German version of WOMAC to patients upon first consultation. Patients with FAI [n=100, mean age 31.7 years, standard deviation (SD) 9.7] and OA (n=57, mean age 60.3 years, SD 11.7) and without comorbidities or prior hip surgery were included and compared to age- and gender-matched control population to FAI (n=200, mean age 32.6 years, SD 5.6). WOMAC data of 157 questionnaires were evaluated by Rasch analysis using RUMM2020 software.

RESULTS

Summation of total WOMAC shows misfit to the Rasch model as well as multidimensionality. While the pain subset shows adequate fit and is unidimensional, item reduction is required to fit a unidimensional subset of functional items to the Rasch model. Summating the two fitting subsets yields again slight model misfit and multidimensionality requiring further item reduction. Finally, a 12-item version of the total WOMAC shows good model fit and unidimensionality, i.e., internal construct validity, for assessment of patients with FAI and OA without differential item functioning (DIF). A person separation index (PSI)=0.93 indicates a high internal consistency reliability for the 12-item subscale. Scores for FAI are significantly higher than control (P<0.001, effect size 0.71) and lower than OA group (P<0.001, effect size 0.45). Adequate statistical power is shown discriminating the three groups, therefore indicating some evidence also for external construct validity.

CONCLUSIONS

The WOMAC as a total construct is multidimensional and summating the subsets into a total score is not valid. The reduced 12-item WOMAC is demonstrated to have internal construct validity for assessing patients with FAI and OA on the same scale and high internal consistency reliability. Discrimination of the groups with adequate statistical power also indicates external construct validity.

Rasch analysis of the Fatigue Severity Scale in multiple sclerosis

BACKGROUND

The 9-item, Fatigue Severity Scale (FSS-9) has been widely used as an outcome measure in multiple sclerosis (MS). Modern psychometric theory, in the form of the Rasch measurement model, has set new quality standards for outcome measures by appraising a broad range of measurement properties in addition to the reliability and validity emphasized by classical test theory.

OBJECTIVE

To appraise the FSS-9 by application of the Rasch model.

METHOD

The FSS-9 was posted to patients with clinically definite MS in two centers in the United Kingdom. Analysis was based on 416 records (55% response).

RESULTS

The 9-item scale failed to meet Rasch model expectations. Two items had poor discrimination across the scale, and two further items showed bias for factors such as age. Removal of these four items provided a valid 5-item Rasch scale that satisfied strict tests of unidimensionality.

CONCLUSION

Summating the nine items of the FSS-9 is invalid. Five items (FSS-5), which seem to be measuring the social impact of fatigue, provide a strictly unidimensional Rasch scale. Studies using the FSS-9 may need to be re-evaluated using the FSS-5, preferably using the Rasch transformed scores.

Literacy affected ability to adequately discriminate among categories in multipoint Likert Scales

OBJECTIVES

To assess the effect of inability to read on a five-point Likert scale, using the WHOQOL-BREF response scale. It is hypothesized that inability to read is related to loss of discriminant power among the five-category response scale.

STUDY DESIGN AND SETTING

In a cross-sectional design, nonreaders (n=141) and subjects educated at a graduate level (n-907) were assessed. Statistical analyses combined classic and modern psychometric approaches (Confirmatory factor analysis and Rasch analysis).

RESULTS

There is a remarkable difference in the psychometric performance of items across the two subgroups. Fit indexes proved to be closer to the ideal for the graduate group, but not for the nonreader group. Reducing the number of response categories improved the model exclusively for the nonreader sample. Nonreaders do not interpret the scale as a true five-category scale, but exhibit a response pattern indicative that only three categories are in fact considered.

CONCLUSION

These results confirm the hypothesis that a multiple-category response scale is not suitable for nonreaders. They suggest that a simpler response scale should be adopted to achieve a more reliable and satisfactory psychometric performance in this population. This effect seems to be stronger when more abstract and subjective constructs are involved.

Validity of the Developmental Test of Visual-Motor Integration Supplemental Developmental Test of Visual Perception

Visual perceptual skills of school-age children are often assessed using the Supplemental Developmental Test of Visual Perception of the Developmental Test of Visual-Motor Integration. The study purpose was to consider the construct validity of this test by evaluating its scalability (interval level measurement), unidimensionality, differential item functioning, and hierarchical ordering of its items. Visual perceptual performance scores from a sample of 356 typically developing children (171 boys and 185 girls ages 5 to 11 years) were used to complete a Rasch analysis of the test. Seven items were discarded for poor fit, while none of the items exhibited differential item functioning by sex. The construct validity, scalability, hierarchical ordering, and lack of differential item functioning requirements were met by the final test version. Since 7 test items did not fit the Rasch analysis specifications, the clinical value of the test is questionable and limited.

Clinical trial: a randomized controlled cross-over study of flupenthixol + melitracen in functional dyspepsia

BACKGROUND

Functional dyspepsia is a prevalent condition associated with diminished quality of life (QoL) and high economic burden.

AIM

To study the efficacy of a combination of flupenthixol and melitracen (F + M) with anxiolytic and antidepressant properties in functional dyspepsia using a randomized controlled cross-over design.

METHODS

Patients met the Rome III criteria for functional dyspepsia and a validated questionnaire was used to exclude those with anxiety or depression. Moreover, patients had to have failed a trial of acid-suppressive therapy and Helicobacter pylori eradication when positive. End points included subjective global symptom relief and QoL assessed by the Nepean Dyspepsia Index (NDI).

RESULTS

Twenty-five patients (14 females, 11 males; mean age = 34.3 +/- 9.9 years) were enrolled and 24 completed the 8-week study. There was a significant improvement in subjective global symptom relief with F + M vs. placebo (ITT: 73.9% vs. 26.1%, P = 0.001) and a significant drop in the NDI score vs. placebo (ITT: -9.0 +/- 11.9 vs. -2.4 +/- 8.9, P = 0.03). No difference was noted whether the initial treatment was F + M or placebo. No significant side effects were noted.

CONCLUSIONS

A combination of F and M is safe and effective in the short-term treatment of functional dyspepsia. F + M is associated with significant improvement in QoL independent of the presence of anxiety or depression.

Goal attainment scaling: current methodological challenges

PURPOSE

To validate the underlying mathematical process of Goal Attainment Scaling (GAS), as an illustration of the problems encountered by patient-generated indexes in the quest of patient-centred outcomes.

METHODS

Data are simulated to represent the type of input to GAS. Rasch analysis is used to linearize the response categories for each variable associated with each goal, thus making it possible to compare the ordinal non-linear outcome of the GAS process with its linear equivalent, under the assumption of strict unidimensionality. Using a minimum clinically important difference (MCID), the level of difference between the two estimates is assessed.

RESULTS

Over 14% of the simulated cases showed a magnitude of difference in change scores between the ordinal and linear-based GAS scores greater than the MCID. These differences were most likely to occur when patients start or finish their GAS scores at the margins of the score range, where non-linearity is greatest. The results show that the GAS process does not support mathematical operations such as multiplication. Apparent clinically meaningful changes scores can be generated solely from the non-linear nature of ordinal scores.

CONCLUSIONS

Using patient-centred approaches to measurements such as GAS presents formidable scientific challenges. Suggestions are made which, in the context of GAS applications, may overcome some of these limitations. This involves the establishment of 'item banks' of goals which can be pre-calibrated onto a unidimensional metric such that linearized versions of the various scores (e.g., difficulty) could be imported into the process.

Measuring the impact of health problems among adults with limited mobility in Thailand: further validation of the Perceived Impact of Problem Profile

Background

The Perceived Impact of Problem Profile (PIPP) was developed to provide a tool for measuring the impact of a health condition from the individual's perspective, using the ICF model as a framework. One of the aims of the ICF is to enable the comparison of data across countries, however, relatively little is known about the subjective experience of disability in middle and low-income countries. The aim of this study was to assess the validity of the Perceived Impact of Problem Profile (PIPP) for use among adults with a disability in Thailand using Rasch analysis.

Methods

A total of 210 adults with mobility impairment from the urban, rural and remote areas of northeast Thailand completed the PIPP, which contains 23 items assessing both impact and distress across five key domains (Self-care, Mobility, Participation, Relationships, and Psychological Well-being). Rasch analysis, using RUMM2020, was conducted to assess the internal validity and psychometric properties of the PIPP Impact subscales. Validation of the PIPP Impact scales was conducted by comparing scores across the different response levels of the EQ5D items.

Results

Rasch analysis indicated that participants did not clearly differentiate between 'impact' and 'distress,' the two aspects assessed by the PIPP. Further analyses were therefore limited to the PIPP Impact subscales. These showed adequate psychometric properties, demonstrating fit to the Rasch model and good person separation reliability. Preliminary validity testing using the EQ5D items provided support for the PIPP Impact subscales.

Conclusion

The results provide further support for the psychometric properties of the PIPP Impact scales and indicate that it is a suitable tool for use among adults with a locomotor disability in Thailand. Further research is needed to validate the PIPP across different cultural contexts and health conditions and to assess the usefulness of separate Impact and Distress subscales.

Development and validation of the Brazilian version of the Attitudes to Aging Questionnaire (AAQ): An example of merging classical psychometric theory and the Rasch measurement model

BACKGROUND

Aging has determined a demographic shift in the world, which is considered a major societal achievement, and a challenge. Aging is primarily a subjective experience, shaped by factors such as gender and culture. There is a lack of instruments to assess attitudes to aging adequately. In addition, there is no instrument developed or validated in developing region contexts, so that the particularities of ageing in these areas are not included in the measures available. This paper aims to develop and validate a reliable attitude to aging instrument by combining classical psychometric approach and Rasch analysis.

METHODS

Pilot study and field trial are described in details. Statistical analysis included classic psychometric theory (EFA and CFA) and Rasch measurement model. The latter was applied to examine unidimensionality, response scale and item fit.

RESULTS

Sample was composed of 424 Brazilian old adults, which was compared to an international sample (n = 5238). The final instrument shows excellent psychometric performance (discriminant validity, confirmatory factor analysis and Rasch fit statistics). Rasch analysis indicated that modifications in the response scale and item deletions improved the initial solution derived from the classic approach.

CONCLUSION

The combination of classic and modern psychometric theories in a complementary way is fruitful for development and validation of instruments. The construction of a reliable Brazilian Attitudes to Aging Questionnaire is important for assessing cultural specificities of aging in a transcultural perspective and can be applied in international cross-cultural investigations running less risk of cultural bias.

A longitudinal evaluation of the Center for Epidemiologic Studies-Depression scale (CES-D) in a rheumatoid arthritis population using Rasch analysis

Background

The aim of this study was to test the internal validity of the total Center for Epidemiologic Studies-Depression (CES-D) scale using Rasch analysis in a rheumatoid arthritis (RA) population.

Methods

CES-D was administered to 157 patients with RA over three time points within a 12 month period. Rasch analysis was applied using RUMM2020 software to assess the overall fit of the model, the response scale used, individual item fit, differential item functioning (DIF) and person separation.

Results

Pooled data across three time points was shown to fit the Rasch model with removal of seven items from the original 20-item CES-D scale. It was necessary to rescore the response format from four to three categories in order to improve the scale's fit. Two items demonstrated some DIF for age and gender but were retained within the 13-item CES-D scale. A new cut point for depression score of 9 was found to correspond to the original cut point score of 16 in the full CES-D scale.

Conclusion

This Rasch analysis of the CES-D in a longstanding RA cohort resulted in the construction of a modified 13-item scale with good internal validity. Further validation of the modified scale is recommended particularly in relation to the new cut point for depression.

ACTIVLIM: A Rasch-built measure of activity limitations in children and adults with neuromuscular disorders

A common measure of activity limitations for both children and adults with neuromuscular disorders was developed using the Rasch model. A self-reported questionnaire containing daily activities was submitted to 245 adult patients and to the parents of 124 affected children from the two major Belgian communities. They were asked to provide their perceived difficulty in performing daily activities on a three-level scale. The 22 items of the final scale define a unidimensional and linear measure of activity limitations and show a continuous progression in their difficulty. The item difficulty hierarchy is invariant with regard to the diagnosis, community, gender and age. The scale exhibits a good precision, since the 22 items are well targeted on our sample (r=0.96); furthermore, it is reproducible over time (ICC=0.93). The patients' measures are related to the Functional Independence Measure motor score (rho=0.85), to the Brooke (rho=-0.63) grade and to the Vignos (rho=-0.83) grade.

The Foot Posture Index: Rasch Analysis of a Novel, Foot-Specific Outcome Measure

OBJECTIVE

To investigate the internal construct validity of a clinician-assessed measure of foot position, the Foot Posture Index (FPI), versions FPI-8 and FPI-6.

DESIGN

Rasch analysis of baseline FPI scores from studies conducted during the development of the instrument.

SETTING

A community-based and a hospital-based study, conducted at 2 institutions.

PARTICIPANTS

Measures were obtained from 143 participants (98 men, 45 women; age range, 8-65y).

INTERVENTIONS

Not applicable.

MAIN OUTCOME MEASURES

Rasch analysis was undertaken using RUMM2020 software in order to evaluate the following properties of the FPI: unidimensionality of each item included in the FPI, the differential item functioning (DIF) of each item, and item and person separation indices.

RESULTS

In the developmental draft of the instrument, the 8-item FPI-8 showed some misfit to the Rasch model (chi(16)(2) test=27.63, P=.03), indicating lack of unidimensionality. Two items were identified as problematic in the Rasch modeling: Achilles' tendon insertion (Helbing's sign), which showed illogical response ordering and "congruence of the lateral border of the foot," which showed misfit, indicating that this item may be measuring a different construct (chi(2)(2) test=15.35, P<.01). All FPI-8 items showed an absence of DIF, and the person separation index (PSI) was good (PSI=.88). The revised FPI-6, which does not include the 2 problematic items, showed unidimensionality (chi(12)(2) test=11.49, P=.49), indicating a good overall fit to the model, and improvement over the preliminary version. With the removal of the 2 problematic items, there were no disordered thresholds; all items remained DIF free and all individual items displayed a good fit to the model. The person-separation index for the FPI was similar for both the 8-item (FPI-8=.880) and 6-item (FPI-6=.884) versions.

CONCLUSIONS

The original FPI-8 showed significant mismatching to the model. The 2 items in the FPI-8 that were identified as problematic in clinical validation studies were also found to be contributing to the lack of fit to the Rasch model. The finalized 6-item instrument showed good metric properties, including good individual item fit and good overall fit to the model, along with a lack of differential item functioning. This analysis provides further evidence for the validity of the FPI-6 as a clinical instrument for use in screening studies and shows that it has the potential to be analyzed using parametric strategies.

Cross-diagnostic validity in a generic instrument: an example from the Functional Independence Measure in Scandinavia

Background

To analyse the cross-diagnostic validity of the Functional Independence Measure (FIM™) motor items in patients with spinal cord injury, stroke and traumatic brain injury and the comparability of summed scores between these diagnoses.

Methods

Data from 471 patients on FIM™ motor items at admission (stroke 157, spinal cord injury 157 and traumatic brain injury 157), age range 11–90 years and 70 % male in nine rehabilitation facilities in Scandinavia, were fitted to the Rasch model. A detailed analysis of scoring functions of the seven categories of the FIM™ motor items was made prior to testing fit to the model. Categories were re-scored where necessary. Fit to the model was assessed initially within diagnosis and then in the pooled data. Analysis of Differential Item Functioning (DIF) was undertaken in the pooled data for the FIM™ motor scale. Comparability of sum scores between diagnoses was tested by Test Equating.

Results

The present seven category scoring system for the FIM™ motor items was found to be invalid, necessitating extensive rescoring. Despite rescoring, the item-trait interaction fit statistic was significant and two individual items showed misfit to the model, Eating and Bladder management. DIF was also found for Spinal Cord Injury, compared with the other two diagnoses. After adjustment, it was possible to make appropriate comparisons of sum scores between the three diagnoses.

Conclusion

The seven-category response function is a problem for the FIM™ instrument, and a reduction of responses might increase the validity of the instrument. Likewise, the removal of items that do not fit the underlying trait would improve the validity of the scale in these groups. Cross-diagnostic DIF is also a problem but for clinical use sum scores on group data in a generic instrument such as the FIM™ can be compared with appropriate adjustments. Thus, when planning interventions (group or individual), developing rehabilitation programs or comparing patient achievements in individual items, cross-diagnostic DIF must be taken into account.

Measuring the impact and distress of health problems from the individual's perspective: development of the Perceived Impact of Problem Profile (PIPP)

Background

The aim of this study was to develop and conduct preliminary validation of the Perceived Impact of Problem Profile (PIPP). Based on the biopsychosocial model of health and functioning, the PIPP was intended as a generic research and clinical measurement tool to assess the impact and distress of health conditions from the individuals' perspective. The ICF classification system was used to guide the structure of the PIPP with subscales included to assess impact on self-care, mobility, participation, relationships and psychological well-being. While the ICF focuses on the classification of objective health and health related status, the PIPP broadens this focus to address the individuals' subjective experience of their health condition.

Methods

An item pool of 23 items assessing both impact and distress on five key domains was generated. These were administered to 169 adults with mobility impairment. Rasch analysis using RUMM2020 was conducted to assess the psychometric properties of each set of items. Preliminary construct validation of the PIPP was performed using the EQ5D.

Results

For both the Impact and Distress scales of the PIPP, the five subscales (Self-care, Mobility, Participation, Relationships, and Psychological Well-being) showed adequate psychometric properties, demonstrating fit to the Rasch model. All subscales showed adequate person separation reliability and no evidence of differential item functioning for sex, age, educational level or rural vs urban residence. Preliminary validity testing using the EQ5D items provided support for the subscales.

Conclusion

This preliminary study, using a sample of adults with mobility impairment, provides support for the psychometric properties of the PIPP as a potential clinical and research measurement tool. The PIPP provides a brief, but comprehensive means to assess the key ICF components, focusing on the individuals' perspective of the impact and distress caused by their health condition. Further validation of its use across different health conditions and varying cultural settings is required.