Hello. Here are some references that may be of help. Katz J, Melzack R. Measurement of pain. Surg Clin North Am. 1999 Apr; 79(2):231-52. Scott, J and Huskisson, E.C., Graphic representation of pain, Pain 2 11976) 175-184.
Carlsson, Anna Maria: Assessment of chronic pain. I. Aspects of the Reliability and Validity of the Visual Analogue Scale: Pain, 16 (1983) 87-101. Breivik EK, Bjornsson GA, Skovlund E. A comparison of pain rating scales by sampling from clinical trial data: Clin J Pain. 2000 Mar:16 (1): 22-8
Onhaus, E.E and Adler R., Methodologic problems in the measurement of pain: a comparison between the verbal rating scale and the visual analog scale, Pain 1 (1975) 379-384.
Steven J. Linton and K. Gunnar Gotestam: A clinical comparison of two pain scales; correlation, Remembering Chronic pain and a measure of compliance: Pain, 17 (1983) 57-65.