Improved understanding of the various test scores found in clinical norm-referenced assessment instruments will enable therapists to make informed decisions about selecting the best test scores for measuring and reporting applied clinical research activities (i.e., initial evaluation, progress report, program evaluation, treatment effectiveness research). The application of norm-referenced test scores in treatment effectiveness research is discussed, with all issues applying directly to the use of individual clients’ test scores in making clinical decisions and reporting assessment results. The scientific principles of measurement used by researchers are outlined, followed by a discussion of the various test scores that can be obtained from a norm-referenced test and how each score is interpreted. Recommendations are provided for the use of the various test scores in treatment effectiveness research, specifically addressing research questions that can be answered from results obtained using the different measures available in norm-referenced tests: Did the clients improve in performance? At what rate are the skills being gained?