Visual Analogue Scale & Statistical Concerns

🔸A frequently used tool in anaesthesia research is the 100 mm visual analogue scale (VAS). 

🔸This 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. 

🔸Because 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. 

🔸If there is some doubt about the sample distribution, then the data should be considered ordinal.

🔸When small numbers of observations are being analysed (say, less than 30 observations), it is preferable to consider VAS data as ordinal. 

🔸For 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.

🔸There has been some controversy in the literature regarding which statistical tests should be used when analysing VAS data. 

🔸Mantha et al surveyed the anaesthetic literature and found that approximately 50% used parametric tests. 

🔸Dexter and Chestnuts used a multiple resampling (of VAS data) method to demonstrate that parametric tests had the greater power to detect differences among groups. 

🔸Myles 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