CDC SVI Frequently Asked Questions (FAQ)
CDC Social Vulnerability Index (CDC SVI) is a tool that uses U.S. Census data to determine the social vulnerability of every census tract. ATSDR’s Geospatial Research, Analysis & Services Program (GRASP) maintains CDC SVI to help public health officials and local planners better prepare for and respond to emergency events like hurricanes, disease outbreaks, or exposure to dangerous chemicals.
Learn more about CDC SVI in the frequently asked questions and responses below.
Documentation for all versions of CDC SVI can be found on our Data & Documentation Download page by selecting the Documentation radio button and specifying the year of interest under the green data banner.
The CDC SVI ranking variables for the four themes are RPL_THEME1 for the Socioeconomic Status theme, RPL_THEME2 for the Housing Composition & Disability theme, RPL_THEME3 for the Minority Status & Language theme, and RPL_THEME4 for the Housing & Transportation theme. The CDC SVI ranking variable for overall vulnerability is RPL_THEMES. See CDC SVI Documentation for further information.
A percentile ranking represents the proportion of tracts (or counties) that are equal to or lower than a tract (or county) of interest in terms of social vulnerability. For example, a CDC SVI ranking of 0.85 signifies that 85% of tracts (or counties) in the state or nation are less vulnerable than the tract (or county) of interest and that 15% of tracts (or counties) in the state or nation are more vulnerable.
In the CDC SVI Interactive Map, we classify data using quartiles (0 to .2500, .2501 to .5000, .5001 to .7500, .7501 to 1.0) and indicate that the classification goes from least vulnerable to most vulnerable. While we do not have required cutoffs for working with CDC SVI data, categorizing CDC SVI values using a quantile classification (i.e. tertiles, quartiles, quintiles, etc.) is common. If you choose to categorize CDC SVI values, we recommend you do so appropriately based on your question of interest.
The U.S.-based CDC SVI database compares the social vulnerability of a census tract (or county) to all tracts (or counties) in the U.S. The state-based CDC SVI database only compares the social vulnerability of a census tract (or county) to all tracts (or counties) within a particular state of interest. This is also why you will find disceprancies when comparing state-based CDC SVI rankings to the CDC SVI Interactive Map, as our map contains the U.S.-based CDC SVI database.
CDC SVI is updated every two years based on U.S. Census Bureau data releases. The Census releases American Community Survey data in December of the year following the Survey. Thus, there is a time differential in when we can produce and disseminate updates of CDC SVI.
ZIP code areas are postal delivery areas, and their borders are subject to change by the postal service. While ZIP code enumeration may change frequently, U.S. Census Bureau ZCTAs are relatively stable representations of ZIP codes for a snapshot of time. However, neither ZIP codes nor ZCTAs are based on homogeneous demographics, as are census tracts. In addition, Census data used in CDC SVI are readily available at census tract and county levels, not at the ZIP code level. Persons interested in using ZIP levels may benefit from incorporating HUD USPS ZIP Code Crosswalk Filesexternal icon.
We provide CDC SVI for Puerto Rico, which is not included in the U.S.-wide CDC SVI and is ranked separately from the U.S. in a Puerto Rico-wide CDC SVI. However, this is the only U.S. territory for which we produce CDC SVI because sociodemographic census variables for the other territories (Guam, American Samoa, the U.S. Virgin Islands, the North Mariana Islands) are unavailable or are not collected at similar geographic resolutions as those required for CDC SVI. In addition, starting with the 2014 version of CDC SVI, we provide CDC SVI rankings for tribal census tracts.