Many chapters in the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides) provide a range of numbers within four or five impairment classes, and evaluating physicians are required to justify the choice of a class and the impairment rating within the range specified for that class. The process of assigning a class is accomplished by an assessment of the patient's disease process, objective findings, activities of daily living (ADLs), and required continuing medical treatment. A table in the article provides hyperlinks to different scales for measurement of instrumental activities of daily living and ADL; use of these standardized instruments increases the reliability of assessing the impact on ADLs. After describing the patient's ability to perform ADLs, the evaluating physician should ensure that the physical examination and laboratory findings lead to the final diagnosis used for rating. To provide an impairment rating, the evaluator must identify and confirm a physiologic abnormality via physical examination or laboratory or x-ray findings consistent with the diagnosis. Impairment rating must be based on a clearly established diagnosis, not solely on subjective complaints from the patient. As with all impairment evaluations, the rating should be based on a thoughtful evaluation and obtaining the data needed to apply the criteria specified in the AMA Guides.