Two fundamental problems in mental test theory are to estimate true score and to estimate the amount of error when testing an examinee. In this report, three probability models which characterize a single test item in terms of a population of examinees are described. How these models may be modified to characterize a single examinee in terms of an item domain is explained. A numerical illustration is given which shows how the modified models might be used to adjust observed scores when comparing examinees. (Author/AL)