CDC/ATSDR SVI Frequently Asked Questions (FAQ)
The CDC/ATSDR Social Vulnerability Index (CDC/ATSDR 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 the CDC/ATSDR 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 the CDC/ATSDR SVI in the frequently asked questions and responses below.
The CDC/ATSDR SVI ranking variables for the four themes are RPL_THEME1 for the Socioeconomic Status theme, RPL_THEME2 for the Household Composition & Disability theme, RPL_THEME3 for the Minority Status & Language theme, and RPL_THEME4 for the Housing Type & Transportation theme. The CDC/ATSDR SVI ranking variable for overall vulnerability is RPL_THEMES. See the CDC/ASTDR 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/ATSDR 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/ATSDR 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/ATSDR SVI data, categorizing CDC/ATSDR SVI values using a quantile classification (i.e., tertiles, quartiles, quintiles, etc.) is a common approach. If you choose to categorize CDC/ATSDR SVI values, we recommend you do so appropriately based on your question of interest.
The U.S.-based CDC/ATSDR 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/ATSDR SVI database compares the social vulnerability of a census tract (or county) solely to tracts (or counties) within a particular state of interest. This is the reason one may find discrepancies when comparing state-based CDC/ATSDR SVI rankings to the CDC/ATSDR SVI Interactive Map, as the map is drawn from the U.S.-based CDC/ATSDR SVI database.
The CDC/ATSDR SVI is updated every two years based on U.S. Census Bureau data releases. The Census usually 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 to CDC/ATSDR 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 the CDC/ATSDR 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/ATSDR SVI for Puerto Rico, which is not included in the U.S.-wide CDC/ATSDR SVI database. These data for Puerto Rico are ranked separately from U.S. states in a stand-alone Puerto Rico-specific CDC/ATSDR SVI database. However, this is the only U.S. territory for which we produce CDC/ATSDR 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 the geographic resolutions required for CDC/ATSDR SVI. In addition, starting with the 2014 version of CDC/ATSDR SVI, we provide CDC/ATSDR SVI data for tribal census tracts.
For more information, please contact the CDC/ATSDR SVI Coordinator (firstname.lastname@example.org).