X-ray screeners have to know which items are prohibited and what they look like in x-ray images of passenger bags (knowledge-based factors). In addition, effective x-ray screening requires the abilities to cope with bag complexity, superposition by other objects, and rotation of objects (image-based factors). Knowledge-based factors are expected to be highly dependent on training whereas image-based factors are related to visual-cognitive abilities and aptitudes . To test to what extend these

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... o factors are influenced by training, 334 screeners took two x-ray screening tests before and after two years of recurrent computer-based training (CBT). The Prohibited Items Test (PIT) measures rather knowledge-based factors, the X-Ray Object Recognition Test (X-Ray ORT) image-based factors. The results showed indeed a much better detection performance in the PIT after two years of training. Thus, CBT can increase the knowledge of prohibited items and what they look like in x-ray images of passenger bags substantially. The increase in detection performance in the X-Ray ORT was much smaller. This indicates that image-based factors are indeed related to visual-cognitive abilities and aptitudes that can be increased by CBT less effectively. The implications for selection and training of x-ray screeners are discussed.