Wolfe J M. et al found that subject's miss rate increased markedly when target prevalence decreased in simulated X-ray luggage screening task, which was so-called the low prevalence effect. He thought it was caused by shift of observer's decision criteria. But the number of trials with target (NTT) also affected the effect. The present study had two experiments, and there were two blocks in each experiment. Subjects in Exp 1 were in different NTT (20 vs. 100) but the same target prevalence (both 50%); In Exp 2, NTT was the same (both 20) but the target prevalence was different (50% vs. 5%). The results showed that subject's miss rate was mainly changed with NTT, and decision criteria was up to the target prevalence, Wolfe's conclusion was not completely correct.