Item discrimination index, which indicates the ability of the item to distinguish whether or not the individuals have acquired the qualities that are evaluated, is basically a validity measure and it is estimated by examining the fit between item score and the test score. Based on the definition of item discrimination index, classroom observation scores were used in this study instead of test scores as the indication of having the tested quality. In the framework of the study, a 25-item multiple-choice test prepared in the context of 8th grade Mathematics Unit "Multipliers and Multiples" was administered to a total of 109 8th graders (44 females, 65 males) studying in 4 separate classrooms of Ömer Nasuhi Bilmen Secondary School in Sanliurfa Province. Furthermore, these students' Mathematics teachers were asked to observe and score students during the unit and the obtained observation scores were used as external criterion in estimating the discrimination index. By using this criterion, fit values estimated with the help of upper and lower groups consisting of 27% from the extremes of the criterion score distribution and biserial correlation were compared with the traditional conditions where test scores were utilized. It was found that item discrimination indices based on classroom observations were higher than those based on test scores in both of the discrimination indices estimated via upper-lower 27% groups and biserial correlation. This finding was discussed to be related to the fact that while classroom observation scores were an external validity criterion, test scores were composed of items whose discrimination values were calculated. The finding also demonstrated that classroom observation scores were more rigid and eliminative than test scores in terms of discrimination.