Adding It All Up

Calculating graduation rates isn't an easy matter, but states are using a variety of methods that critics say obscure the extent of the problem.

To the uninitiated, calculating a high school’s graduation rate probably seems easy. Just divide the number of graduates by the total number of students in a class.

But accounting for the constant change that high schools experience—students who transfer out or in, others who drop out only to return later, and still others who need five years to earn a diploma—makes producing accurate rates far from easy.

Pressure to come up with more accurate numbers has been mounting, though, because of the federal No Child Left Behind Act. The 2001 reauthorization of the Elementary and Secondary Education Act judges the performance of high schools and districts based, in part, on their graduation rates. The provision was intended to balance the law’s focus on raising test scores with a requirement that would discourage schools from...