The U.S. Department of Education used, for the first time, a common metric to calculate graduation rates for all states and the District of Columbia. The statistics, released Monday, reveal the rate at which first-time ninth-grade students earn a high-school diploma within four years. Iowa had the highest graduation rate at 88%, while the rate in Washington, D.C., was the lowest at 59%. Many states reported lower graduation rates than revealed in past calculations, and the data also show gaps in achievement among students.