The pain visual analog scale: Is it linear or nonlinear?

The visual analog scale (VAS) is a tool widely used to measure pain, yet controversy surrounds whether the VAS score is ratio or ordinal data. We studied 52 postoperative patients and measured their pain intensity using the VAS. We then asked them to consider different amounts of pain (conceptually twice as much and then half as much) and asked them to repeat their VAS rating after each consideration (VAS(2) and VAS(3), respectively). Patients with unrelieved pain had their pain treated with IV fentanyl and were then asked to rate their pain intensity when they considered they had half as much pain. We compared the baseline VAS (VAS(1)) with VAS(2) and VAS(3). The mean (95% confidence interval) for VAS(2:1) was 2.12 (1.81-2.43) and VAS(3:1) was 0.45 (0.38-0.52). We conclude that the VAS is Linear for mild-to-moderate pain, and the VAS score can be treated as ratio data. Implications: A change in the visual analog scale score represents a relative change in the magnitude of pain sensation. Use of the VAS in comparative analgesic trials can now meaningfully quantify differences in potency and efficacy.