The Impact of Image Based Factors and Training on Threat Detection Performance in X-Ray Screening

In this study, two experiments are reported which investigated the relative importance of five different image based factors and one human factor (training) in mediating threat detection performance of human operators in airport security x-ray screening. Experiment 1 was based on a random sample of roughly 16’000 records of threat image projection (TIP) data. TIP is a software function available on state-of-the-art x-ray screening equipment that allows the projection of fictional threat images (FTIs) into x-ray images of passenger bags during the routine baggage screening operation. Analysis of main effects showed that image based factors can substantially affect screener detection performance in terms of the hit rate (identification of FTIs). There were strong effects of FTI view difficulty (rotation of FTIs) and superposition of FTIs by other objects in the x-ray image of a passenger bag. The amount of opacity in the x-ray image of a passenger bag had a small although significant effect on detection performance. The two image based factors clutter and bag size did not have a significant effect. Experiment 2 was conducted using an offline-test in order to provide controlled and more detailed data for analyzing the image based factors from Experiment 1, as well as the human factor of training. In particular the individual factors’ main effects on detection performance, main effects of all factors taken together and factor interactions were analyzed. In the test design the following image-based factors were varied systematically: Threat (FTI) category (guns, knives, improvised explosive devices, other threats), view difficulty, superposition, bag complexity (a combination of opacity and clutter) and bag size. Data were collected from 200 screening officers at five sites across Europe. For screener training all five sites use the same computer-based training system. Consistent with the results obtained in Experiment 1, there were large main effects of threat (FTI) category, view difficulty, and superposition. Again consistent with Experiment 1, effects of bag complexity (opacity and clutter) and bag size were much smaller. In addition to Experiment 1, the number of computer based training (CBT) hours was available for each security officer participating in the study. Training turned out to be a key driver to improving threat detection performance in x-ray screening and seemed to mediate the effects of some image based factors. Possible implications regarding the enhancement of human-machine interaction in x-ray screening are discussed