The use of the Visual Analogue Scale (VAS) in rehabilitation outcomes.

The Visual Analogue Scale (VAS) is a common form of response option in health outcome studies, often used to measure pain, amongst other things, and is generally presented as a single line of 100 mm with anchor words at either end (e.g. no pain – worst possible pain). It was first published in the early 1920's (1, 2) though not widely used at that time (3). There are variations on the VAS theme, sometimes with shorter lines (e.g. 65 mm), and whatever the length, while they are typically presented in horizontal format, they can also be presented as vertical lines (4), and the two forms of presentation have been considered as equivalent (5). Another alternative is where it is presented as a thick line of blocks (looking much like bricks laid end-to-end) with 11 categories, ranging from 0–10, which is called a Numeric Rating Scale (NRS). VAS and NRS formats have also been shown to be equivalent (6, 7). The VAS has a long history of use in medical outcome studies, and is ubiquitous across all specialities , including Physical and Rehabilitation Medicine (8–10). It is used either in the form of a single Item scale (e.g. for pain), or as a type of response option for multiple item scales (11,12). A VAS is considered to reduce the confounding effect of variation between individual interpretations of the graduations used for rating scales; is preferred by participants who perceive their desired response as not corresponding with rating scale graduations (16) and enables a finer distinction between subjective states to be made (3). However, it has also been found that patients find it difficult to judge how to rate their pain on the pain VAS line, finding it 'not very accurate', 'sort of random', 'almost guesswork' or having to 'work it into numbers first' (17). Consequently, in some studies, very low test-retest reliability has been reported (18). An implicit assumption made by the majority of people who use a VAS (or NRS) in either their clinical practice or research, is that the scale is interval in nature, or even ratio (7, 19–21). Thus, all sorts of mathematical and parametric procedures are applied. For example Bland and Altman plots have been used together with intra-class correlation coefficient (ICC), two-way mixed effects analysis of variance model with patients random and rating method fixed (7). Means and standard deviations are all commonly reported for VAS …