Historically, the average difference between the actual values and the projections of current expenditures, current expenditures per pupil, and teacher salaries has been about 2 percent for projections that are 2 or 3 years out from the year of the last actual data. Projections for years that are further out from the last year with actual data tend to be less accurate. The average difference between the actual values and projections 7 or more years out from the last year with actual data generally has been over 4 percent for current expenditures and current expenditures per pupil, and over 8 percent for teacher salaries.
Long-term projections that are economically based, such as projections of current expenditures and teacher salaries, are generally less accurate than long-term demographic projections, such as projections of elementary and secondary enrollment. Recent NCES projections of current expenditures generally have been less accurate than recent NCES projections of public elementary and secondary enrollment, but more accurate than projections of teacher salaries. Projections of teacher salaries generally have been less accurate than projections of public elementary and secondary enrollment, and similar in accuracy to projections of first-professional degrees. See appendix A for further discussion of the accuracy of recent projections of current expenditures and teacher salaries, and see table A2 in appendix A for the mean absolute percentage errors (MAPEs) of these projections.
