Relevance of visual inspection strategy and knowledge about everyday objects for X-ray baggage screening

Abstract

The screening of passenger bags at airports can be understood as a visual inspection task that consists of visual search and decision. Security officers (screeners) visually search for prohibited items in X-ray images and decide whether secondary search (e.g. using manual search or explosive trace detection) is needed. A screener's decision can be explained with signal detection theory and its measures (hit rate, false alarm rate, sensitivity and decision criterion). In this experiment tested whether a specifically instructed visual inspection strategy can influence the hit and false alarm rate. In addition, it was investigated whether knowledge about the visual appearance of harmless everyday objects in X-ray images is relevant for the detection of prohibited items. To this end, 31 screeners of an international airport conducted a simulated X-ray baggage screening task with two different instructions (normal vs. liberal decision) on how to conduct visual inspection: In the normal decision condition, screeners were instructed to visually inspect the X-ray images like they were used to from their job. In the liberal decision condition, screeners were instructed to visually analyze each object in the X-ray image and only decide that the bag was harmless if each object in the image could be recognized as harmless The screeners knew half of the prohibited items from computer-based training while the other half were novel prohibited items. In addition, knowledge about the visual appearance of everyday objects in X-ray images was measured. The results show that screeners were able to change their decision criterion depending on the instructed visual inspection strategy. Knowledge about harmless everyday objects was positively associated with detection performance and most notably correlated with the hit rate for novel threat items in the liberal decision condition. Implications for improving X-ray screening at airports using a risk-based and adaptive approach are discussed.