Presented is an overview of the 1979 Illinois Inventory of Educational Progress (IIEP) for eighth-grade mathematics. The IIEP is a systematic effort by the Illinois State Board of Education to collect information on the educational achievement of Illinois students in certain areas and to make that information available to educational decision-makers. The IIEP employs an objective-referenced approach, with desired student performance expressed in terms of objectives. Students to be tested are selected in a two-stage random sampling method. Since the IIEP is geared towards determining how groups of Illinois students perform on given tasks, no individual student, teacher, school, or district is identified. As part of the study, teachers of participating students were asked to estimate the percentage of students who would obtain correct answers to individual test items. Of five objectives containing three or more items, teacher estimates were higher than student performance in all cases, but the difference was only statistically significant for one objective. (MP)