National dropout rates have fluctuated between 4 percent and 6.7 percent annually over the past 30 years, according to the National Center for Education Statistics.

In 2001, the most recent year available, the dropout rate was about 5 percent.

However, differences in the way such statistics are kept and other variables make it very difficult to make comparisons across states or to compare Hawai'i's aggregate four-year dropout rate to the national rate, education experts say.

Figured the way NCES computes the national rate, Hawai'i's annual dropout rate has fluctuated between 3 percent and 4 percent on average since 2001, according to the state Department of Education.

Through the 1990s, the annual dropout rate in Hawai'i was closer to 5.3 percent, according to NCES statistics, which include students repeating the ninth grade, as well as transfer students and military students who may leave before graduation.

However, because it offers better tracking of each student, the Hawai'i DOE figures its dropout rates by cohort — comparing the number of students who have left the school system to those who began together in ninth grade four years earlier.

Using this method, DOE statisticians calculate a dropout rate of approximately 15 percent for each four-year class since 2001-02.

But the science of tracking students is inexact, and the data do not account for as many as 500 students annually who leave the state or change schools within the state.

Even with those discrepancies, the public schools have lost between 6,000 and 8,700 students — dropouts from the past five graduating classes, according to DOE statistics and projections.

Nationally, few states are able to create cohort dropout rates because they don't have a centralized educational system as Hawai'i does.

According to the definition used by the National Center for Education Statistics, dropouts are the number of students in a given school year who neither complete high school by earning a diploma nor receive a diploma through an alternative program.

Reach Beverly Creamer at bcreamer@honoluluadvertiser.com.