Abstract
ObjectivesPain visual analogue scales (VAS) are commonly used in clinical trials and are often treated as an interval level scale without evidence that this is appropriate. This paper examines the internal construct validity and responsiveness of the pain VAS using Rasch analysis.MethodsPatients (n = 221, mean age 67, 58% female) with chronic stable joint pain (hip 40% or knee 60%) of mechanical origin waiting for joint replacement were included. Pain was scored on seven daily VASs. Rasch analysis was used to examine fit to the Rasch model. Responsiveness (Standardized Response Means, SRM) was examined on the raw ordinal data and the interval data generated from the Rasch analysis.ResultsBaseline pain VAS scores fitted the Rasch model, although 15 aberrant cases impacted on unidimensionality. There was some local dependency between items but this did not significantly affect the person estimates of pain. Daily pain (item difficulty) was stable, suggesting that single measures can be used. Overall, the SRMs derived from ordinal data overestimated the true responsiveness by 59%. Changes over time at the lower and higher end of the scale were represented by large jumps in interval equivalent data points; in the middle of the scale the reverse was seen.ConclusionsThe pain VAS is a valid tool for measuring pain at one point in time. However, the pain VAS does not behave linearly and SRMs vary along the trait of pain. Consequently, Minimum Clinically Important Differences using raw data, or change scores in general, are invalid as these will either under- or overestimate true change; raw pain VAS data should not be used as a primary outcome measure or to inform parametric-based Randomised Controlled Trial power calculations in research studies; and Rasch analysis should be used to convert ordinal data to interval data prior to data interpretation.
Highlights
Visual analogue scales (VAS) are commonly used in clinical trials and other studies as primary [1,2,3] or secondary outcomes [4,5] or as a tool to derive a health utility index [6]
Data followed a normal distribution at baseline and follow-up
Fit to the Rasch model The baseline pain VAS scores were tested against the Rasch model, which had a satisfactory fit the person fit residual standard deviation (SD) was high (1.7)
Summary
Visual analogue scales (VAS) are commonly used in clinical trials and other studies as primary [1,2,3] or secondary outcomes [4,5] or as a tool to derive a health utility index [6]. The VAS is a 10 cm long straight line, marked at each end with labels which anchor the scale [7]. In the context of pain, patients are asked to place a mark on the line at a point representing the severity of their pain where the anchors are ‘no pain’ and ‘pain as bad as it could be’ (labels vary between studies). Just because clinicians and researchers assume that the score in millimetres is interval in nature, this does not necessarily mean that patients score it as an interval scale. Some research suggests that patients find it difficult to judge how to rate their pain on the pain VAS line [10,11], finding it ‘not very accurate’, ‘sort of random’, ‘almost guesswork’ or having to ‘work it into numbers first’ [10]. A study on business travellers revealed that scores on a VAS (in this case 76mm in length) cluster into much smaller groups [12]
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