Metropolitan statistical area (MSA)

A geographic entity based on a county or a group of counties with at least one urbanized area with a population of at least 50,000 and adjacent counties with economic ties to the central area. The economic ties are measured by commuting patterns. Similar standards are used to define micropolitan statistical areas, except that the urban clusters are smaller, with populations between 10,000 and 49,999. Counties that are not classified as metropolitan or micropolitan statistical areas are outside the federal Office of Management and Budget (OMB) classification. Non-MSAs include urban populations not located within an MSA as well as completely rural areas. OMB defines, or delineates, metropolitan and micropolitan statistical areas by applying published standards to data. A new set of standards is published once every 10 years, in the year of the decennial census. Then 3 years later, OMB issues the list of metropolitan and micropolitan statistical areas and their county components after the standards have been applied using census and commuting data. For more information about the delineation files, see the Delineation Files webpage. Between the once-a-decade comprehensive review of statistical area standards and delineations, OMB issues periodic updates reclassifying counties to reflect current population data. For more information, see the Metropolitan and Micropolitan webpage. Updated standards, available from the 2020 Standards for Delineating Core Based Statistical Areas webpage were published in July 2021 but have not yet been enacted. The 2010 standards are currently used to define MSA in Health, United States. See OMB’s 2010 Standards for Delineating Metropolitan and Micropolitan Statistical Areas. The 2013 OMB bulletin with county classifications is available from OMB BULLETIN NO. 13-01. Data systems adopt MSA standards sometime after they are published. The adoption of a new set of standards or subsequent reclassifications may create a discontinuity in trends.

National Health Interview Survey (NHIS)

The place of residence of respondents to NHIS is classified as metropolitan or nonmetropolitan. Since the 2016 data release, the metropolitan variable has been based on the 2013 county classifications.

National Immunization Surveys (NIS)  (last updated June 2023)

Starting with 2010 data, NIS includes an MSA variable based on the respondent’s location of residence. If county of residence is not provided during the household interview, additional household-reported address information is used. If incomplete address information is available, MSA status is imputed. The method for determining MSA depends on the sampling frame (landline or cell phone). For landline telephone numbers, MSA is assigned based on the telephone exchange. For cell phone numbers, MSA is imputed using a hot-deck process, in which each missing value is replaced with an observed response from a similar unit. The OMB county classifications used are updated regularly. For example, 2019 data are based on the August 2017 delineation file.