Other: All public-use files available were downloaded. The downloaded files include documentation about programs and services for HS English learners, school safety and discipline, condition of public school facilities, dual credit and exam-based courses, arts education surveys of elementary and secondary school teachers, dropout prevention services and programs surveys, distance education courses for public elementary and secondary school students, alternative schools and programs for public school students at risk of educational failure, teachers' useof educational technology in US public schools, after-school programs in public elementary schools, internet access in US public schools, distance education courses for public elementary and and secondary school students, public school principals' perceptions of their school facilities, foods and physical activity in public elementary schools, district survey of alternative schools and programs,condition of public school facilities, occupational programs and the use of skill competencies at the secondary and post-secondary levels, national student service-learning and community service survey, and advanced telecommunications in US private schools. However, all listed documentations were not available for all listed years.