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Even during a time of robust recovery, the national unemployment rate rarely drops below 5%. However, in a period in which unemployment in the United States is 5.8%, five states have rates under 4%.

The five states are Minnesota (3.9%), North Dakota (2.8%), Nebraska (3.4%), South Dakota (3.3%) and Utah (3.3%).

Even a year ago, these states had rates well below the national average.

One of the things the five states have in common is small populations. South Dakota and North Dakota are two of the three smallest states by population. Nebraska is 37th and Utah is 33rd. Only Minnesota is of any significant size based on population, 21st in the nation. Another thing the states have in common is their vast sizes. Minnesota is the 12th largest state base on square mileage. Utah is 13th, Nebraska is 16th, South Dakota is 17th and North Dakota is 19th.

Each of the states has businesses that are “recession proof” to some extent. Minnesota is the headquarters of well over a dozen of the largest companies in the United States. These include Target Corp. (NYSE: TGT), Best Buy Co. Inc. (NYSE: BBY), General Mills Inc. (NYSE: GIS) and Cargill.

South Dakota has benefited from government spending and agriculture. Nebraska has been helped by a similar economy. The shale industry dominates North Dakota’s economy. In Utah, petroleum production and mining are critical part of the state gross domestic product.

The lessons: be big geographically, small in population and rely on a small number of industries.

Notes: BLS methodology, unemployment by state and unemployment by region.

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