 ACT publishes a technical manual that summarizes studies conducted on its validity in predicting freshman GPA, equating different high school GPAs, and measuring educational achievement.Colleges use the ACT and the SAT because there are substantial differences in funding, curricula, grading, and difficulty among U.S. secondary schools due to American federalism, local control, the prevalence of private, distance, homeschooled students, and lack of a rigorous college entrance examination system similar those used in some other countries.ACT scores are used to supplement the secondary school record and help admission officers put local data—such as coursework, grades, and class rank—in a national perspective.