Conjoint measurement and Stevens' psychophysical function are used to examine the relationship between the probability of death (or threat to life) and two injury severity measurements, i.e., AIS and ISS. It is found that threat to life is essentially the sole criterion used in deriving the AIS scores, and the functional relationship between ISS and fatality rates can be well explained by Stevens' psychophysical function. It is suggested that the probability of death be used directly to indicate the severity of injury, or as the sole criterion in deriving the AIS. The joint probability of death can then be calculated as a measure of the total severity of multiple injuries.