Because of the recent popularity of the three-parameter logistic model among the researchers who apply latent trait theory, it will be worthwhile to investigate the effect of noise accommodated in different models. In the present paper, four types of models on the dichotomous response level, Types A, B, C and D, are considered. Type A does not include noise, and the other three types do. Observations are made as to how much total item information is lost because of the noise, how the item response information functions are affected, how the speed of convergence to the normality of the conditional distribution of the maximum likelihood estimate of ability, given a specific ability level, is affected, and so forth. (Author)