Metropolitan Statistical Areas (MSA) partition counties into two categories: Metropolitan and non-Metropolitan. Metropolitan areas are composed of a core containing a large population nucleus and adjacent communities that have a high degree of integration with the core. In this system, counties with cities or urbanized areas of over 50,000 residents and a total population of at least 100,000 are classified as Metropolitan. Outlying counties meeting a complex set of commuting and population characteristics are also designated Metropolitan. Although the remaining, non-Metropolitan areas are often considered to be rural, this is not entirely correct, because these counties may include substantial population concentrations.
A county-based system such as MSA, that attempts to describe the diversity in settlement patterns in a relatively large area by a single number, may not provide an informative depiction. A county may be designated as Metropolitan even though only a small portion is urbanized and the rest is distinctly rural. However, because county boundaries don't change much, every county will be represented by a measure, even after an extended period of time.
MSA were developed at the Office of Management and Budget (OMB). They are based on population and commuting information from the 1990 census and are defined according to the OMB 1993 Metropolitan definitions. PL_MSA1993 is included on the HCUP file because of the widespread use of this measure in the past, but it has now been superseded by Core-Based Statistical Areas (CBSA), which are available as PL_CBSA.