When they apportion impairment in musculoskeletal cases, evaluators encounter a variety of unique issues and problems. The first step in apportionment is scientifically based causation analysis. Arbitrary or opinion-based unscientific apportionment estimates that amount to little more than speculation should be avoided. The AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), Fourth and Fifth Editions, are similar in their assessment of orthopedic impairment, but significant differences exist between these and the Sixth Edition. Individuals may experience impairments on several occasions. For example, if the first injury was rated using an earlier edition of the AMA Guides and a second injury occurs and is rated using a more current edition, then the most recent edition in the current jurisdiction is used to recalculate the rating for the first injury. Regarding which edition of the AMA Guides to use, evaluators should be aware of the jurisdictional requirements and also the timing to ensure that the individual is at maximum medical improvement. If the issue to be determined is apportioning the cause of the injury and not the impairment rating, then different criteria are used and the AMA Guides to the Evaluation of Disease and Injury Causation, Second Edition, is an invaluable resource. Extensive sidebars discuss qualitative vs qualitative apportionment and steps that evaluators can take to ensure that body regions and conditions are not confused (ie, that an apples-to-apples comparison is taking place).