States are grappling with a federal requirement that is forcing them to use a new, more uniform method of calculating high school graduation rates--a method that, in some states, is yielding rates that are 20 percentage points lower than those states have reported in the past. Under a 2008 update to federal education rules, the states were required to replace their patchwork of graduation-rate formulas with a four-year "cohort" rate, beginning in the 2010-2011 school year, and to use that number this school year to determine whether schools are making adequate progress under the No Child Left Behind Act. The new rate is part of an effort to improve both the consistency and accuracy of graduation-rate records across the country and to move closer to allowing for state-by-state comparisons. As the 2008 regulations themselves note, "establishing a uniform and more accurate measure of calculating graduation rate that is comparable across states is a critical and essential step forward in improving high school accountability." The four-year adjusted cohort rate set out in the regulations requires states to track individual students and capture how many first-time 9th graders in a given class proceed to graduate with a standard diploma four years later. That method is considered more accurate than previous methods but often yields a graduation rate that is lower than the results of states' old formulas. By way of comparison, the so-called "leaver rate," which was at one point in use in 32 states, calculates a percentage to represent the number of students who graduate with a standard diploma compared with the sum of those students, students who receive an alternative completion credential, and students who have dropped out over the course of high school. Students who do not fit one of those school-leaving categories can fall through the cracks using this formula. Another commonly used formula, the "averaged freshman graduation rate," calculates the number of students who graduate with a standard diploma in four years but relies on an estimate rather than an exact number of 9th graders. The new calculation means that the graduation rate may appear dramatically different even if the number of students who actually graduate hasn't changed.