Apportionment sometimes is necessary when a person had a prior injury that affected the same organ system or when there was evidence of a pre-existing disorder that was similar to the injury that will be rated. The past three editions of the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides) include subtle differences in the way that apportionment is defined. In addition to the information provided in the appropriate edition, certain philosophical and mechanistic considerations should be assessed before any apportionment and then should be used consistently in any subsequent cases that require apportionment. The first step in apportionment is a scientifically based causation analysis. Evaluators must avoid arbitrary, opinion-based unscientific apportionment estimates that are little more than speculations. In a situation that involves multiple injuries, if the impairment assessment has reached a defensible position with respect to causation then the rating for the first injury is determined using the edition of the AMA Guides that was used to rate the second injury. For example, if the rating for a second injury will be determined using the sixth edition, then the data for the first injury should be reworked using the sixth edition rules, irrespective of the rating for the first injury and the edition that was used.