An impairment rating is a percentage figure indicating the amount of functional loss an individual has. This figure is reached using the methodologies and figures of the American Medical Association's Guides to the Evaluation of Permanent Impairment. A combination of diagnosis, functional data, clinical examination and results of diagnostic studies is used to reach a final figure for an extremity, limb, or for whole person. An impairment rating can take into account the amount of impairment likely to be present with pre-existing chronic conditions. This allows clearer determination of the accident related element of the claim