It is not widely recognized that the VAS has measurement properties that are superior to other commonly used scales such as the NRS. Unlike VAS, the 11 point-NRS definitely does not have ratio scale properties and has no distinct zero point [Fig. 4 of Ref. 13]. Compared to VAS ratings, NRS ratings have been shown to be artificially higher for both clinical and experimental pain (13). The notion that NRS ratings can easily substitute for VAS ratings because they are highly correlated with each other is very misguided. For example, both are monotonic functions of heat stimulus intensity and are likely to be highly correlated, yet the 11 point-NRS stimulus-response curve is displaced above the VAS curve. Only the latter reflects accurate ratios or proportions of pain intensity and appears to have a true zero point (3,13). Given the superior psychometric characteristics of VAS, it is astonishing that NRS has been recommended over other pain scales, including VAS, in clinical research and practice (14,15). Because of their measurement advantages, studies that use mechanical VAS to conduct sensory tests on patients with neuropathic pain, fibromyalgia syndrome, and IBS pain are emphasized in this chapter. Special emphasis will be placed on measuring and characterizing IBS pain.
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