Clinicians can use the base rates of low scores in healthy people to reduce the likelihood of misdiagnosing cognitive impairment. In the present study, base rates were developed for the Wechsler Adult Intelligence Scale-Fourth Edition (WAIS-IV) and Wechsler Memory Scale-Fourth Edition (WMS-IV) using 900 healthy adults and validated on 28 patients with moderate or severe traumatic brain injuries (TBIs). Results indicated that healthy people obtain some low scores on the WAIS-IV/WMS-IV, with prevalence rates increasing with fewer years of education and lower predicted intelligence. When applying the base rates information to the clinical sample, the TBI patients were 13 times more likely to be identified as having a low cognitive profile compared with the controls. Using the base rates information is a psychometrically advanced method for establishing criteria to determine low cognitive abilities on the WAIS-IV/WMS-IV. (Contains 4 tables.)