An analysis of projection errors from similar models used in the past sixteen editions of Projections of Education Statistics that contained expenditure projections indicates that mean absolute percentage errors (MAPEs) for total current expenditures in constant dollars were 1.4 percent for 1 year out, 2.3 percent for 2 years out, 2.9 percent for 5 years out, and 3.6 percent for 10 years out. MAPEs for current expenditure per pupil in current dollars were 1.4 percent for 1 year out, 2.2 percent for 2 years out, 3.3 percent for 5 years out, and 5.5 percent for 10 years out.
Long-term projections that are economically based, such as projections of current expenditures, 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 bachelor's degrees. See appendix A for further discussion of the accuracy of recent projections of current expenditures, and see table A-2 in appendix A for the mean absolute percentage errors (MAPEs) of these projections.