Because persons with mental retardation cannot be executed for murder, the diagnosis becomes a life and death matter. The American Association on Mental Retardation (now the American Association on Intellectual and Developmental Disabilities) and other associations agree that IQ alone is an insufficient criterion and adaptive functioning also needs to be considered. However, there is no satisfactory quantitative measure including both IQ and adaptive functioning. We propose a solution by defining a total quotient (TQ) scale, a composite of both IQ and standardized adaptive functioning scores. We estimate the margin of error in such scores (IQ, or adaptive functioning, or TQ) is 10 points, four times the usual one SD value given by intelligence test developers. The procedure here renders moot the distinction between convergent and divergent validity.