A method for determining the severity of a presentation of cognitive impairment has been developed and scientifically validated specifically for use with cases in which the causative diagnosis is traumatic brain injury. The method determines if the impairment, as demonstrated on neuropsychology testing, is consistent with the severity of the claimed cause. Such scientific linkage of an objective measurement of impairment severity and the severity of the cause is quite rare. The research related to this method did not consider methodology for impairment rating presented in the AMA Guides to the Evaluation of Permanent Impairment (AMA Guides), so this article compares the new method to the impairment rating methodology from the AMA Guides, Sixth Edition, Section 13.3d, Mental Status, Cognition, and Highest Integrative Function. This article offers an example of how neuropsychology test results for an individual case can be applied to the classes of impairment that are included in Table 13-8, Criteria for Rating Neurologic Impairment Due to Alteration in Mental Status, Cognition, and Highest Integrative Function. This method has the advantages of standardization, objectivity, and being based on scientific findings that make the unique contribution of actually linking severity of impairment to severity of the causative factors. The authors caution that the method is only a subset of the methodology from Section 13.3d and does not justify abandonment of the remainder of this section of the AMA Guides.