Presented is an overview of the 1979 Illinois Inventory of Educational Progress (IIEP) for fourth-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 education 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, or school 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 the eight measured objectives, it was found that teacher estimates were significantly higher than student performance for only one objective. (MP)