GRAND FORKS (AP) - North Dakota lost about $1 billion over 14 years because people who left the state tended to earn more when they left than people coming into the state, the state Data Center reports.

A comparison of individual federal tax returns from 1993 to 2006 shows people moving into North Dakota brought a total of $6.1 billion in net taxable income, while people moving out took with them $7.1 billion in taxable income, state Data Center director Richard Rathge said.

Grand Forks County saw the biggest loss, with a net outflow of $274 million in net taxable income over those years due to migration, Rathge said. He attributed it to the Red River flood of 1997 and cuts at the Grand Forks Air Force Base.

Ward County, which includes Minot and the Air Force base near there, had the second biggest net outflow, of $175 million, he said.

The Data Center report said Burleigh County saw a net gain of $20 million, likely helped by state government and a growing energy industry. Morton County and Bottineau County also gained.

"We need to need to keep in mind that the IRS data have important limitations, such as the need to match returns between the years and misreporting due to address changes," Rathge said in a statement. "Nonetheless, these data offer us one of the few ways we can document the economic consequences of migration."