Anchor-based approaches compare the change in PRO score to some other measure of change, considered an anchor or external criterion. Very few studies have relied on an objective external criterion. For instance, Farrar et al. [2] compared patients’ ratings on pain scales to the objective amount of ingested pain medication. Most commonly, studies compare PRO scores to the patients’ answers to another subjective assessment, typically a Global Assessment Rating in which the patients rate themselves to some extent as ‘‘better,’’ ‘‘unchanged,’’ or ‘‘worse.’’