A nationwide study in 2005 of 2100 cases referred for impairment rating review found 80% to be erroneous, and 89% of these erroneous ratings were higher than appropriate. Among whole person erroneous ratings (839 of 1037 cases critiqued), the original physician's rating averaged 15.5% whole person permanent impairment, but following rerating by physician experts, the corrected rating averaged 5.6%; only 7% of the cases were underrated. All ratings were based on the AMA Guides to the Evaluation of Permanent Impairment, (AMA Guides), Fifth Edition. Tables and figures show error rates according to portion of the body affected, expert vs original rating, and other explanatory variables. Two physicians who use the AMA Guides should arrive at similar conclusions about impairment ratings, but most physicians have not received instruction about assessing impairment, disability, or causation and lack an adequate ability to assess these issues. Causation requires a given cause and a given effect that are associated with a reasonable degree of medical probability and also requires documentation with appropriate scientific evidence (not self-reports or historical time frames). Those who prepare and review assessments of impairment should ensure that clinical causation assessments were accurate, that the rating was performed at maximal medical improvement, that examination findings were consistent, and that the individual's normal state was determined.