CDC SVI Documentation 2010

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CDC SVI 2010 Documentation – 3/31/2015

Introduction

What is Social Vulnerability?

Every community must prepare for and respond to hazardous events, whether a natural disaster like a tornado or a disease outbreak, or an anthropogenic event such as a harmful chemical spill. The degree to which a community exhibits certain social conditions, including poverty, car ownership, or number of persons in households, may affect that community’s ability to prevent human suffering and financial loss in the event of disaster. These factors describe a community’s social vulnerability.

What is the Social Vulnerability Index?

ATSDR’s Geospatial Research, Analysis & Services Program (GRASP) has created a tool to help public health officials and emergency response planners identify and map the communities that will most likely need support before, during, and after a hazardous event.

The Social Vulnerability Index (SVI) indicates the relative vulnerability of every U.S. Census tract. Census tracts are subdivisions of counties for which the Census collects statistical data. The SVI ranks the tracts on 14 social factors, including unemployment, lack of vehicle access, and crowded housing, and further groups them into four related themes. Thus, each tract receives a ranking for each Census variable and for each of the four themes, as well as an overall ranking.

How can the SVI help communities be better prepared for hazardous events?

The SVI provides specific socially and spatially relevant information to help public health officials and local planners better prepare communities to respond to emergency events such as severe weather, floods, disease outbreaks, or chemical exposure.

The SVI can be used to:

• Allocate emergency preparedness funding by community need.
• Estimate the amount and type of needed supplies like food, water, medicine, and bedding.
• Help decide how many emergency personnel are required to assist people.
• Identify areas in need of emergency shelters.
• Plan the best way to evacuate people, accounting for those who have special needs, such as those without vehicles, the elderly, or people who do not understand English well.
• Identify communities that will need continued support to recover following an emergency or natural disaster.

Important Notes on the SVI Database

• Keep the data in geodatabase format. Converting to shapefile changes the field names.
• Tracts with zero population for 100% counts were removed during the calculation process. These tracts were added back to mapped data and are shown with a TOTPOP field value of 0. All other numeric fields for zero population tracts were set to -999.
• For tracts with > 0 TOTPOP, a value of -999 in any field either means the value was unavailable from the original census data or we could not calculate a value because of unavailable data.
• Any cells with a -999 were not used for further calculations. For example, total flags do not include fields with a -999 value.
• See the Methods section below for further details.
• Questions? Please visit the SVI web site at http://svi.cdc.gov for additional information

SVI Data Dictionary

*Variables beginning with “E_” are estimates. Variables beginning with “M_” are margins of error for those estimates. Some variables in the database are not listed here; these are generally locational information.

**For a detailed description of SVI 2000 methods, see A Social Vulnerability Index for Disaster Management pdf icon[PDF – 2 MB].

Methods

Variables Used

2010 tract level data

Census 2010 100% count data (SF1) for the following variables:

• Persons aged 65 and older
• Persons aged 17 and younger
• Single parent households with children under 18
• Minority status (i.e. Total population minus white, non-Hispanic population)
• Persons living in Group Quarters

Raw data values for each variable, for each tract, are included in the database.

American Community Survey (ACS), 2006-2010 (5-year) data for the following variables/estimates:

• Persons below the poverty level
• Civilian unemployed
• Per capita income
• No high school diploma for persons aged 25 and older
• Persons who speak English “less than well”
• Housing units with 10 or more units in the structure
• At the household level, more people than rooms
• Mobile homes
• No vehicle access

Raw data estimates for each variable, for each tract, are included in the database. In addition, the margins of error (MOEs) for each estimate are also included.

The US Census Bureau did not collect tract level disability data, included in SVI 2000, for either the 2010 Census or the 2006-2010 ACS. Therefore, a disability variable is not included in SVI 2010.

Proportion calculations

We processed SF1 data using the same method as SVI 2000.**

• We calculated the proportion for each variable for each tract (e.g. proportion of persons aged 65 and older) and included these proportions in the database.
• We used appropriate SF1 variables as denominators (e.g. total population to calculate proportion of persons aged 17 and younger).

Because of estimate error, the ACS data include additional data fields.

• Margins of error (MOEs) are included for each estimate, including derived estimates. We calculated MOEs for derived estimates using Census specifications.*** (See A Compass for Understanding and Using. American Community Survey Data. What General Data Users Need to Knowpdf iconexternal icon.) The confidence level is at the Census standard of 90%.
• We used appropriate ACS estimates as denominators (e.g. total population estimate to calculate the proportion of persons who speak English “less than well”).
• Note: Confidence intervals can be calculated by subtracting the MOE from the estimate (lower limit) and adding the MOE to the estimate (upper limit).

** For a detailed description of SVI 2000 methods, see A Social Vulnerability Index for Disaster Management pdf icon[PDF – 2 MB].

***The ACS Toolbox can be used to calculate MOEs for derived values. Please visit the SVI web site at http://svi.cdc.gov for additional information.

Rankings

We ranked Census tracts within each state and the District of Columbia, to enable mapping and analysis of relative vulnerability in individual states. We also ranked tracts for the entire United States against one another, for mapping and analysis of relative vulnerability in multiple states, or across the U.S. as a whole. Tract rankings are based on percentiles, as for SVI 2000. Percentile ranking values range from 0 to 1, with higher values indicating greater vulnerability.

For each tract, we generated percentile rankings for 1) the fourteen individual variables, 2) the four themes, and 3) an overall ranking.

Theme rankings:  For each of the four themes, we summed the percentiles for the variables comprising each theme. We ordered the summed percentiles for each theme to determine theme-specific percentile rankings.

Overall tract rankings:  We summed the sums for each theme, ordered the tracts, and then calculated overall percentile rankings. Please note; taking the sum of the sums for each theme is the same as summing individual variable rankings.

Flags

We processed data using the same method as SVI 2000. Tracts in the top 10%, i.e. at the 90^th percentile of values, are given a value of 1 to indicate high vulnerability. Tracts below the 90^th percentile are given a value of 0.

For a theme, the flag value is the number of flags for variables comprising the theme. We calculated the overall flag value for each tract as the number of all variable flags.

View print only PDF of CDC SVI 2010 Documentation pdf icon[PDF – 235 KB]