Abstract: In the educational and psychological tests, there exist guessing phenomena that the low-ability examinee makes correct answers on the difficult items, and the sleeping phenomena that the high-ability examinee makes wrong answers on easy items, which would lead to errors. Many researches have been done on the issue of these phenomena, and various methods have been proposed to rectify the overestimation and underestimation. Usually when these methods are used, these responses on items that appear to be too easy or difficult for the examinee need to be down-weighted or omitted. However, responses of the examinees need not to be changed with the four parameter logistic model in which c parameter reflects the guessing phenomena and rectifies the overestimation, γ parameter reflects the sleeping phenomena and rectifies the underestimation. In the personality testes, there exist some items which have non-zero lower asymeptote or non-one upper asymeptote. Four-parameter model could better fit the response data for the test and raise accuracy.

Key words: IRT, guessing phenomenon, sleeping phenomenon, four-parameter model