Census Data & Emergency Preparedness

Special Population Estimates for Impacted Counties in the Gulf Coast Area: Methodology

The impact of Hurricanes Katrina and Rita in late August and early September of 2005 on the population of affected areas is without precedent. In an attempt to provide data users with some information on population size, the Census Bureau produced a special set of January 1, 2006 total household population estimates for the 117 counties in the Gulf Coast Area that were severely impacted by the hurricanes. These 117 counties were those eligible to receive Individual and Public Assistance (IPA) based on the disaster declarations issued by the Federal Emergency Management Agency (FEMA) through October 7, 2005 for Hurricane Katrina and through October 20, 2005 for Hurricane Rita.

To produce these January 1, 2006 estimates of the total household population, the Census Bureau designed a special methodology to modify the routine demographic estimates for the impacted counties. Because of the unique nature of the methodology and data employed, the resulting estimates are not part of the official estimates series. Rather, these special estimates for January 1, 2006, are designed to provide information and indications of the impacts of the events on the population size of the impacted counties.

The special population estimates are a blend of the Census Bureau’s extrapolated household population estimates to January 1, 2006 without any impact of the hurricanes and cumulative net migration estimates as of January 1, 2006, derived from a special Hurricane Katrina Change of Address File from the United States Postal Service (USPS).

Extrapolated Household Population Estimates without Impact of Hurricanes

The Intercensal Population Estimates Program produces annual estimates of the resident and group quarters (GQ) population for all counties using an administrative records-based demographic components of change methodology. (For a detailed description of the data and methodology for county estimates, see http://www.census.gov/popest/counties.) The July 1, 2005 set of estimates of total household population for counties released in March 2006, provide the base for the extrapolations to January 1, 2006 without the impact of the hurricanes. As a first step, the household populations for July 1, 2004 and July 1, 2005 are produced by subtracting the GQ population from the resident population in both years. The July 1, 2005 household population is extrapolated to January 1, 2006 by taking one-half of the numeric difference between the 2004 and 2005 household populations and adding it to the July 1, 2005 household population.

Cumulative Net Migration Estimates

The USPS provided a special Hurricane Katrina Change of Address File that contains the current address for residents of the Katrina-affected areas who, through January 2006, have chosen to report a “change of address” to the USPS. We have used tabulations from this file together with Census 2000 data and made several simplifying and conservative assumptions to develop an estimate of in-migrants to and out-migrants from the affected counties.

The combination of the extrapolated January 1, 2006 estimates without the hurricane impacts and the cumulative net migration estimates developed from the special USPS file produced the special January 1, 2006 estimates.

Limitations

Because of the unique nature of the methodology and data employed, the resulting estimates are not part of the official estimates series. Rather, these special estimates for January 1, 2006, are designed to provide information and indications of the impacts of the events on the population size of the impacted counties. The official sets of population estimates for July 1, 2006, that begin with the release in late December 2006 of total state population, will include the measured cumulative impact through June 30, 2006, of these unprecedented events.