The presence of multiple ratable entities, including symptoms, physical findings, test results, diagnoses, and/or procedures, complicates impairment evaluation; further, patients may have several findings in the same anatomic area and/or findings unrelated to the condition being rated. In complex cases involving a final extremity or whole person impairment (WPI), the examining physician must consider all possible ratable impairments, discarding duplicative or mutually exclusive ratings and converting, adding, or combining to obtain the final rating. Failure to follow the instructions detailed in the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides) is a common source of error in impairment rating. The AMA Guides provides multipliers and, for the upper extremity, tables to convert a body part impairment to an extremity and then to a whole person rating. Jurisdictional requirements vary, but rating physicians commonly are asked to provide a single impairment percentage for the limb or whole person; when two or more impairments are involved, the rater must add or combine the percentages, carefully following the specific instructions in the AMA Guides. Adding two ratings is no different than deriving any other arithmetic sum, but impairment percentages much more commonly are combined than added to ensure that, no matter how many impairments are present, the total is never greater than 100% loss of an extremity or 100% WPI.