Data from certified screeners performing an x-ray inspection task for 4 hours, or 1000 images, were analyzed to identify the nature of the vigilance decrement. The expected vigilance decrement was found, with performance measured by probability of detection (PoD) and probability of false alarm [P(FA)] decreasing from hour 1 to hour 4. Correlations between PoD and P(FA) indicate that sensitivity between hours remained the same, however a shift in criterion (Beta) occurred. Significant decreases in both detection and stopping time were found from the first hour to the second, third, and fourth hour. Evidence of changes in the search component of the time per item was found to account for part of the vigilance decrement. As the task continued, participants spent less time actively searching the image, as opposed to other activities. Evidence is provided for truncation of active search as security inspection continues.
Data obtained utilizing image enhancements in a carry-on bag x-ray screening task were analyzed to determine whether and how image enhancements affect performance. To complement earlier studies of experienced screeners, sixty-six novices to the screening task used six different overall enhancements. Results indicated a significant worsening of performance, A', between Original images and Negative images, but no performance differences for the other enhancements, similar to effects found for experienced screeners. There was little overall performance learning taking place on the task. More detailed analysis by splitting response times into search and non-search components revealed little more enhancement effect, but a reduction in False Alarm response time as the task progressed. It appears that the locus of lack of positive effects of enhancements is not just a function of familiarity with the current screen view.
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