๐ธA frequently used tool in anaesthesia research is the 100 mm visual analogue scale (VAS).
๐ธThis is most commonly used to measure postoperative pain, but can also be used to measure a diverse range of (mostly) subjective experiences such as preoperative anxiety, postoperative nausea, and patient satisfaction after ICU discharge.
๐ธBecause there are infinite possible values that can occur throughout the range 0-100 mm, describing a continuum of pain intensity, most researchers treat the resulting data as continuous.
๐ธIf there is some doubt about the sample distribution, then the data should be considered ordinal.
๐ธWhen small numbers of observations are being analysed (say, less than 30 observations), it is preferable to consider VAS data as ordinal.
๐ธFor a number of practical reasons, a VAS is sometimes converted to a 'verbal rating scale', whereby the subject is asked to rate an endpoint on a scale of 0-10 (or 0-5), most commonly recorded as whole numbers. In this situation it is preferable to treat the observations as ordinal data.
๐ธThere has been some controversy in the literature regarding which statistical tests should be used when analysing VAS data.
๐ธMantha et al surveyed the anaesthetic literature and found that approximately 50% used parametric tests.
๐ธDexter and Chestnuts used a multiple resampling (of VAS data) method to demonstrate that parametric tests had the greater power to detect differences among groups.
๐ธMyles et al have recently shown that the VAS has properties consistent with a linear scale, and thus VAS scores can be treated as ratio data. This supports the notion that a change in the VAS score represents a relative change in the magnitude of pain sensation. This enhances its clinical application.
Reference: Statistical Methods for Anaesthesia and Intensive Care, Paul S Myles and Tony Gin
