High school graduation rates in the District, long thought to be inflated by generous counting methods, will be adjusted downward by about 20 percent under a new, more accurate formula, officials at the Office of the State Superintendent of Education confirmed this week.

It means that the 2010 completion rate of 73 percent — just under the national average — will be closer to fifty percent when the 2011 data are released within the next few days. A study last year by Education Week placed the city’s 2008 graduation rate at 43 percent.

States and school districts have been historically slipshod about counting who finishes. Under the old formula, OSSE divided the number of graduating seniors by that number plus the sum of those who left in each of the preceding four years. But the U.S. Department of Education now requires the “adjusted cohort” method, which will use improved data collection methods to track every individual student from ninth to 12th grade, accounting for factors such as transfers and years repeated.

The expected 20 percent drop was first reported by WAMU.