Optimising multi-layered security screening

Screening for prohibited items at airports is an example of a multi-layered screening process. Multiple layers of screening – often comprising different technologies with complementary strengths and weaknesses – are combined to create a single screening process. The detection performance of the overall system depends on multiple factors, including the performance of individual layers, the complementarity of different layers, and the decision rule(s) for determining how outputs from individual layers are combined. The aim of this work is to understand and optimise the overall system performance of a multi-layered screening process. Novel aspects include the use of realistic profiles of alarm distributions based on experimental observations and a focus on the influence of correlation/orthogonality amongst the layers of screening. The results show that a cumulative screening architecture can outperform a cascading one, yielding a significant increase in system-level true positive rate for only a modest increase in false positive rate. A cumulative screening process is also more resilient to weaknesses in the individual layers. The performance of a multi-layered screening process using a cascading approach is maximised when the false positives are orthogonal across the different layers and the true positives are correlated. The system-level performance of a cumulative screening process, on the other hand, is maximised when both false positives and true positives are as orthogonal as possible. The cost of ignoring orthogonality between screening layers is explored with some numerical examples. The underlying software model is provided in a Jupyter Notebook as supplementary material.