Dividing an impairment rating between the current injury and any pre-existing or unrelated causes.
Apportionment is the process of attributing a portion of an impairment to causes other than the injury in question, such as a pre-existing condition, prior injury, or natural aging. Where apportionment applies, the prior impairment is typically calculated and deducted, leaving only the share attributable to the current injury. Not all cases can be apportioned, and rules vary by jurisdiction.
