The application of the processes defined in the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides) should result in reliable impairment ratings, ie, reproducible impairment ratings when the AMA Guides is applied appropriately. This article reviews some of the causes of erroneous impairment ratings based on misapplication of the fifth edition. The first two chapters of the AMA Guides provide the principles of assessing impairment, a key of which is that physicians must be independent and unbiased. Treating physicians cannot be unbiased because they inherently have a patient advocacy role, and they may use approaches that will increase the patient's impairment rating. Clinical and causation errors are common in evaluations; the greatest source of error is examiner inexperience. Evaluations should take place only when the patient is at maximum medical improvement, and evaluators must distinguish between impairment related to the alleged injury and that due to other injury, degenerative disease, or illness, not self-reports. A box shows common causes of erroneous impairment ratings based on the fifth edition. Both the rating physician and the party requesting the rating should recognize the high likelihood of error and should become knowledgeable about the AMA Guides, and the evaluating physician should have had formal training regarding use of the AMA Guides and should possess demonstrated competency in assessing impairment.