SEATTLE — Comparing graduation rates from state to state, or even school to school, can be difficult because all kinds of methods are used to determine them.

Federal officials have a solution that could make that process easier — and more accurate — within the next five years.

U.S. Education Secretary Margaret Spellings in April proposed new rules requiring states to assign students a unique ID number to track the individual from ninth grade through graduation, or until that student drops out.

The proposal, which mirrors an agreement states made with the National Governors Association, would provide every district with a more scientific graduation rate.

Washington state assigned a unique ID to every student four years ago. The class of 2008 will be the first with a graduation rate based on the method Spellings wants mandated for all states.

State officials don't know if the new method will help or hurt Washington's steady 70 percent on-time graduation rate, said Joe Willhoft, director of assessment for the Washington Office of the Superintendent of Public Instruction.

But the point is to come up with a true number, Willhoft said.

The federal government has offered grants to state education departments to improve their data systems, and the money may be used to pay for a system to track students by unique IDs, said U.S. Education Department spokesman Chad Colby.

The government gave a total of $62.2 million to 13 states in 2007 for data systems.

New York is in the process of adopting the new approach. State officials expect the more accurate numbers will be much lower in some cases, because many schools used an index that didn't account for students who dropped out in ninth and 10th grades.

Under No Child Left Behind, states may use their own methods of calculating graduation rates and set their own goals for improving them. Spellings' proposal would tighten a loophole of the federal law.

According to Data Quality Campaign, a national organization encouraging state policymakers to collect and share education data, only a handful of states haven't begun the switch to a more accountable system.

In 2005, all 50 states signed the National Governors Association's "graduation rate compact," pledging to adopt accurate and consistent graduation measurement.

Spellings' proposal mandates calculating graduation rates by following each ninth-grader for four years in every state by the 2013-14 school year.

Some states want the federal government to recognize the students who take longer to complete their diploma.

The National Association of Secondary School Principals would like to see the new rules change the goal to graduation within five years, give equal weight to a GED high-school equivalency diploma and allow special-needs students until age 21 to graduate on time.