Good correlation has been shown between pain scores derived from 4 different rating scales. The correlation was maintained when presentation of the scales was separated by a series of questions and by physical examination. There is good evidence that the 4 scales are measuring the same underlying pain variable as they calibrate well. There is also evidence that an 11-point (0-10) numerical rating scale performs better than both a 4-point simple descriptive scale or a continuous (visual analogue) scale.