The Midwestern United States (also called the Midwest, the Middle West, and The Heartland) is one of the four geographic regions within the United States of America that are officially recognized by the United States Census Bureau. The region consists of twelve states: Illinois, Indiana, Iowa, Kansas, Michigan, Minnesota, Missouri, Ohio, Nebraska, North Dakota, South Dakota, and Wisconsin. A 2006 Census Bureau estimate put the population at 66,217,736. Both the geographic center of the contiguous U.S. and the population center of the U.S. are in the Midwest. The United States Census Bureau divides this region into the East North Central States (essentially the Great Lakes States) and the West North Central States. Chicago is the largest city in the region, followed by Detroit and Indianapolis. Sault Ste. Marie is the oldest city in the region, having been founded in 1668. The term Midwest has been in common use for over 100 years. Other designations for the region have fallen into disuse, such as the "Northwest" or "Old Northwest" (from Northwest Territory) and "Mid-America". Since the book Middletown appeared in 1929, sociologists have often used Midwestern cities (and the Midwest generally) as "typical" of the entire nation. The region has a higher employment-to-population ratio (the percentage of employed people at least 16 years old) than the Northeast, the West, the South, or the Sun Belt states.