New "No Child Left Behind" legislation requires states and school districts that receive Title I funds to participate in biennial NAEP assessments of reading and mathematics in grades 4 and 8. In non-required subjects, some states mandate NAEP participation and some leave the option to participate to local school districts. The schools in the NAEP sample are selected to be representative of the reporting unit or population subgroup. Within a selected school, all students in the appropriate grades have an equal chance of being selected.