The U.S. Department of Education defines the high school graduation rates as follows:  "The adjusted cohort graduation rate (ACGR) is the percentage of public high school freshmen who graduate with a regular diploma within 4 years of starting 9th grade. Students who are entering 9th grade for the first time form a cohort for the graduating class. This cohort is "adjusted" by adding any students who subsequently transfer into the cohort and subtracting any students who subsequently transfer out, emigrate to another country, or die."

The U.S. Department of Education’s Office of Elementary and Secondary Education began collecting the ACGR in 2010-2011 and approved a timeline extension for states to begin reporting 4-year ACGR data, resulting in the 4-year ACGR not being available in one or more of the school years in some states (such as Kentucky, Oklahoma, and Idaho).