This manual presents 12 guidelines to help teachers of students with mental retardation make more informed decisions about instruction by gathering and using student performance data. Development of the guidelines incorporated both teachers' recommendations and researchers' findings about what procedures work best, and guidelines were tested in classrooms for students with moderate and severe disabilities. The guidelines address such concerns as: frequency, quantity, and type of data to collect; graphing; frequency of data review; and application of data to solve learning problems. Three case studies illustrate how special education teachers have used the guidelines to assess student progress and solve instructional problems. The case studies show teachers interpreting data, forming hypotheses about learning problems, and selecting program changes. Six appendices offer a resource list, notes for experienced teachers, a sample anecdotal data chart, a problem analysis worksheet and directions, an outline of recommended instructional changes by type of data path, a list of potential instructional changes, and a summary of the research foundation for the guidelines. (Contains 14 references.) (DB)