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The visual analogue WOMAC 3.0 scale--internal validity and responsiveness of the VAS version.

Kersten P, White PJ, Tennant A - BMC Musculoskelet Disord (2010)

Bottom Line: Therefore, measuring outcome associated with OA is important.The scaling characteristics of sets of items which use VAS require further analysis, as it would appear that they can lead to spurious levels of responsiveness and scale compression because they exaggerate the distortion of the ordinal scale.UKCRN study ID: 4881 ISRCTN78434638.

View Article: PubMed Central - HTML - PubMed

Affiliation: School of Health Sciences, University of Southampton, Highfield, Southampton, UK. P.Kersten@soton.ac.uk

ABSTRACT

Background: Many people suffer with Osteoarthritis (OA) and subsequent morbidity. Therefore, measuring outcome associated with OA is important. The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) has been a widely used patient reported outcome in OA. However, there is relatively little evidence to support the use of the Visual Analogue Scale (VAS) version of the scale. We aimed to explore the internal validity and responsiveness of this VAS version of the WOMAC.

Methods: Patients with chronic hip or knee pain of mechanical origin, waiting for a hip or knee joint replacement completed the WOMAC as part of a study to investigate the effects of acupuncture and placebo controls. Validity was tested using factor analysis and Rasch analysis, and responsiveness using standardised response means.

Results: Two hundred and twenty one patients (mean age 66.8, SD 8.29, 58% female) were recruited. Factor and Rasch analysis confirmed unidimensional Pain and Physical Functioning scales, capable of transformation to interval scaling and invariant over time. Some Differential Item Functioning (DIF) was observed, but this cancelled out at the test level. The Stiffness scale fitted the Rasch model but adjustments for DIF could not be made due to the shortness of the scale. Using the interval transformed data, Standardised Response Means were smaller than when using the raw, ordinal data.

Conclusions: The WOMAC Pain and Physical Functioning subscales satisfied unidimensionality and ordinal scaling tests, and the ability to transform to an interval scale. Some Differential Item Functioning was observed, but this cancelled out at the test level and, by doing so, at the same time removed the disturbance of unidimensionality. The scaling characteristics of sets of items which use VAS require further analysis, as it would appear that they can lead to spurious levels of responsiveness and scale compression because they exaggerate the distortion of the ordinal scale.

Trial number: UKCRN study ID: 4881 ISRCTN78434638.

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Related in: MedlinePlus

Person-Item Threshold Response Map for the WOMAC Physical Functioning subscale (pre-data). The graph displays the person-item threshold distribution map with the x-axes displaying location or difficulty of item thresholds (lower half) and location or level of physical functioning reported by participants (upper half). The y-axes display the frequencies of item thresholds (lower half) and participants (upper half). Data for this figure represent the unbiased person estimates derived from Analysis 2 (see also Table 2) which combined biased items 1 and 5 into a testlet and left the remaining items are unchanged.
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Figure 2: Person-Item Threshold Response Map for the WOMAC Physical Functioning subscale (pre-data). The graph displays the person-item threshold distribution map with the x-axes displaying location or difficulty of item thresholds (lower half) and location or level of physical functioning reported by participants (upper half). The y-axes display the frequencies of item thresholds (lower half) and participants (upper half). Data for this figure represent the unbiased person estimates derived from Analysis 2 (see also Table 2) which combined biased items 1 and 5 into a testlet and left the remaining items are unchanged.

Mentions: As with the pain scale, the PF scale (Rasch transformed scores) had a limited distribution (figure 2) and the ordinality of the raw score was accentuated. For example, a change in 25 points out of a total of 850 (17 items each ranging from 0-50 as scores were halved) at the margins of the raw total (ordinal) VAS physical functioning subscale scores is reflected in a real, interval equivalent change of 311 points (622 mm) (table 4). By contrast, a change of 25 ordinal points (50 mm) in the middle of the scale is in actual fact a change of only 2.2 interval points (4.4 mm).


The visual analogue WOMAC 3.0 scale--internal validity and responsiveness of the VAS version.

Kersten P, White PJ, Tennant A - BMC Musculoskelet Disord (2010)

Person-Item Threshold Response Map for the WOMAC Physical Functioning subscale (pre-data). The graph displays the person-item threshold distribution map with the x-axes displaying location or difficulty of item thresholds (lower half) and location or level of physical functioning reported by participants (upper half). The y-axes display the frequencies of item thresholds (lower half) and participants (upper half). Data for this figure represent the unbiased person estimates derived from Analysis 2 (see also Table 2) which combined biased items 1 and 5 into a testlet and left the remaining items are unchanged.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2874767&req=5

Figure 2: Person-Item Threshold Response Map for the WOMAC Physical Functioning subscale (pre-data). The graph displays the person-item threshold distribution map with the x-axes displaying location or difficulty of item thresholds (lower half) and location or level of physical functioning reported by participants (upper half). The y-axes display the frequencies of item thresholds (lower half) and participants (upper half). Data for this figure represent the unbiased person estimates derived from Analysis 2 (see also Table 2) which combined biased items 1 and 5 into a testlet and left the remaining items are unchanged.
Mentions: As with the pain scale, the PF scale (Rasch transformed scores) had a limited distribution (figure 2) and the ordinality of the raw score was accentuated. For example, a change in 25 points out of a total of 850 (17 items each ranging from 0-50 as scores were halved) at the margins of the raw total (ordinal) VAS physical functioning subscale scores is reflected in a real, interval equivalent change of 311 points (622 mm) (table 4). By contrast, a change of 25 ordinal points (50 mm) in the middle of the scale is in actual fact a change of only 2.2 interval points (4.4 mm).

Bottom Line: Therefore, measuring outcome associated with OA is important.The scaling characteristics of sets of items which use VAS require further analysis, as it would appear that they can lead to spurious levels of responsiveness and scale compression because they exaggerate the distortion of the ordinal scale.UKCRN study ID: 4881 ISRCTN78434638.

View Article: PubMed Central - HTML - PubMed

Affiliation: School of Health Sciences, University of Southampton, Highfield, Southampton, UK. P.Kersten@soton.ac.uk

ABSTRACT

Background: Many people suffer with Osteoarthritis (OA) and subsequent morbidity. Therefore, measuring outcome associated with OA is important. The Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) has been a widely used patient reported outcome in OA. However, there is relatively little evidence to support the use of the Visual Analogue Scale (VAS) version of the scale. We aimed to explore the internal validity and responsiveness of this VAS version of the WOMAC.

Methods: Patients with chronic hip or knee pain of mechanical origin, waiting for a hip or knee joint replacement completed the WOMAC as part of a study to investigate the effects of acupuncture and placebo controls. Validity was tested using factor analysis and Rasch analysis, and responsiveness using standardised response means.

Results: Two hundred and twenty one patients (mean age 66.8, SD 8.29, 58% female) were recruited. Factor and Rasch analysis confirmed unidimensional Pain and Physical Functioning scales, capable of transformation to interval scaling and invariant over time. Some Differential Item Functioning (DIF) was observed, but this cancelled out at the test level. The Stiffness scale fitted the Rasch model but adjustments for DIF could not be made due to the shortness of the scale. Using the interval transformed data, Standardised Response Means were smaller than when using the raw, ordinal data.

Conclusions: The WOMAC Pain and Physical Functioning subscales satisfied unidimensionality and ordinal scaling tests, and the ability to transform to an interval scale. Some Differential Item Functioning was observed, but this cancelled out at the test level and, by doing so, at the same time removed the disturbance of unidimensionality. The scaling characteristics of sets of items which use VAS require further analysis, as it would appear that they can lead to spurious levels of responsiveness and scale compression because they exaggerate the distortion of the ordinal scale.

Trial number: UKCRN study ID: 4881 ISRCTN78434638.

Show MeSH
Related in: MedlinePlus