Malec JF, et al. - For versions completed by individuals with traumatic brain injury (TBI) and an observer, authors here aimed to develop a more precise metric for the Neuropsychiatric Inventory (NPI) Irritability and Aggression scales using all behavioral item ratings for use with individuals with TBI and address the dimensionality of the represented behavioral domains. In this work, irritability and aggression appeared to represent different levels of severity of a single construct. The derived Rasch metric offered a measure of this construct based on responses to all specific items that were appropriate for parametric statistical analysis and could have utility in research and clinical assessments of individuals with TBI.