Found this on the ND state website:
Legislation enacted during 2001 moved North Dakota income tax away from a percentage of the federal income tax liability, replacing it with one that is based on a series of tax brackets. The new tax brackets range from 2.1% (under $27,050 single, or $45,200 married) to 5.54% (on income over $297,350). Under the new law, this is the primary method taxpayers will use when determining their state income tax obligations