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
