Although the U.S. Census Bureau prepares population estimates for MSAs, these statistical units are defined by county borders. Because Massachusetts counties are relatively large and may contain several urban centers, MSAs are an imprecise way to describe the state's urban clusters. For example, Lawrence, Lowell, and Brockton all have closer economic ties with neighboring towns than they do with one another. The Lowell region draws commuters from nearby New Hampshire who might not consider commuting all the way to Boston. Yet these areas are all part of the Boston MSA. Similarly, the cities of Leominster and Fitchburg form the core of a distinct urban cluster. Because they lie within Worcester County, however, they are considered part of the Worcester MSA.