This research compared the two-parameter and three-parameter item-response-models based on the data and theory, and showed the tendencies of those two models. Two-parameter model overestimated the information of the subjects whose ability value were estimated to low, and underestimates the information of the subjects whose ability value were estimated to high. Three-parameter model overestimated the information of the subjects whose ability value were estimated to high, and underestimates the information of the subjects whose ability value were estimated to low. Moreover, because the three-parameter model cannot distinguish whether the correct answer is carried out by the item unique necessary ability or guessing, in an item with low discrimination parameter and difficulty parameter, the guessing parameter becomes bigger. Furthermore, the mutual influence among the guessing parameter, the discrimination parameter and the difficulty parameter were also examined.
