A simulated X-ray luggage screening task was used in this study to find out how to reduce the miss rate of dangerous objects, and how the visual complexity, payoff and so on affected the low prevalence effect (miss rate was lager as target prevalence was lower). The thesis consisted of three parts: In the fist part, I was trying to find out the influence on the miss rate and the low prevalence effect by changing some dimensions of visual complexity, such as items overlapping, noise background, number of interference items, and the contrast of opacity between target and interference items, etc.; the second part was to seek reasons that producing the low prevalence effect in the aspect of procedure. And the last part discussed whether the different methods of payoff and tips of visual saliency could reduce the miss rate, in order to find out a practical and feasible way to reduce the miss rate. The results showed that: (1) Varying some dimensions of visual complexity, such as overlapping, noise background, number of interference items, contrast of opacity between target and interference items, could change the miss rate, but could not eliminate the low prevalence effect. And items overlapping had more influence on miss rate and decision criteria than noise background; (2) The low prevalence effect could not be completely explained by the change of decision criteria; the number of trials with target or the total number of trials should be considered when studied how the target prevalence affected the miss rate; (3) Payoff which used in this study could not reduce the miss rate or cure the low prevalence effect; The tips of visual saliency can reduce the miss rate, so that it could be applied to the real X-ray baggage screening.
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