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In this correspondence, the human factors involved in the design of effective homeland security threat detection systems are described and illustrated for radiation portal monitor (RPM) systems deployed at U.S. ports of entry. Due to the occurrence of nuisance alarms based on naturally occurring radioactive material and the low base rate of nuclear smuggling incidents, it is shown that the probability of a true threat alarm for these systems is extremely low. Receiver operating characteristic analysis of RPM systems illustrates good simple detection capability, but threat classification performance only at the chance level. Application of the human factors concept of the threat likelihood display, based on energy spectrum and cargo commodity data fusion for signal classification, reduces nuisance alarms and increases the probability of a true threat alarm to potentially effective levels. Thus, threat likelihood displays offer an approach for enhancing the effectiveness of homeland security detection and warning systems by raising the credibility of the alerts that are provided.