Appendix A: Description of Methodology cont'd
Calculations of Populations-at-Risk
Presently county-specific measurements of the number of persons with chronic lung disease and other chronic conditions are not generally available. (The primary exception to this is asthma, as state-specific estimates for adult asthma are available through one national survey discussed below.) In order to assess the magnitude of lung disease and other chronic conditions at the state and county levels, we have employed a synthetic estimation technique originally developed by the U.S. Bureau of the Census. This method uses age-specific national estimates of self-reported lung disease and other conditions to project the prevalence of disease by county.
Population Estimates
The U.S. Census Bureau estimated data on the total population of each county in the United States for 2005. The Census Bureau also estimated the age-specific breakdown of the population by county.
Prevalence Estimates
Chronic Bronchitis, Emphysema, Pediatric Asthma and Cardiovascular Disease. In 2005, the National Health Interview Survey (NHIS) estimated the nationwide annual prevalence of diagnosed chronic bronchitis at 8.91 million; the nationwide lifetime prevalence of emphysema was estimated at 3.79 million. The NHIS estimates the prevalence of diagnosed pediatric asthma to be over 6.5 million under age 18. The NHIS estimates the prevalence of cardiovascular disease (CV) at 68 million among adults aged 18 years and over, which includes coronary heart disease, hypertension, stroke, angina pectoris or heart attack.
Due to the revision of the Health Interview Survey questionnaire, prevalence estimates from the American Lung Association State of the Air: 2000 cannot be compared to later publications. Estimates for chronic bronchitis and emphysema can be compared to the State of the Air reports for 2001 through 2007. Furthermore, estimates for chronic bronchitis and emphysema cannot be summed since they represent different types of prevalence estimates.
Pediatric asthma prevalence estimates from this year’s report can only be compared to those in the State of the Air reports since 2004 and not the State of the Air reports from 2000 through 2003 due to a change in the National Health Interview Survey.
Local area prevalence of chronic bronchitis, emphysema, pediatric asthma and CV disease is estimated by applying age-specific national prevalence rates from the 2005 NHIS to age-specific county-level resident populations obtained from the U.S. Census Bureau website. Prevalence estimates for chronic bronchitis, emphysema and CV disease are calculated for those aged 18-44, 45 to 64 and 65+. The prevalence estimate for pediatric asthma is calculated for those under age 18.
Adult Asthma. In 2005, the Behavioral Risk Factor Surveillance System (BRFSS) survey indicated that approximately 8.0 percent of adults residing in the United States reported currently having asthma. The information on adult asthma obtained from the Behavioral Risk Factor Surveillance System survey cannot be compared with pediatric asthma estimates that are derived from the National Health Interview Survey.
The prevalence estimate for adult asthma is calculated for those aged 18 to 44, 45 to 64 and 65+. Local area prevalence of adult asthma is estimated by applying age-specific state prevalence rates from the 2005 BRFSS to age-specific county-level resident populations obtained from the U.S. Census Bureau website.
Diabetes Estimates. In 2005, the National Health Interview Survey estimated the nationwide lifetime prevalence of diabetes at 16.2 million. Local area prevalence of diabetes is estimated by applying age-specific national prevalence rates from the 2005 NHIS to age-specific county-level resident populations obtained from the U.S. Census Bureau website. Prevalence estimates for diabetes are calculated for those aged 18-44, 45 to 64 and 65+.
Limitations of Estimates. Since the statistics presented by the NHIS and the BRFSS are based on a sample, they will differ (due to random sampling variability) from figures that would be derived from a complete census, or case registry of people in the U.S. with these diseases. The results are also subject to reporting, non-response and processing errors. These types of errors are kept to a minimum by methods built into the surveys.
Additionally, a major limitation of both surveys is that the information collected represents self-reports of medically diagnosed conditions, which may underestimate disease prevalence since not all individuals with these conditions have been properly diagnosed. However, the NHIS is the best available source that depicts the magnitude of chronic disease on the national level, and the BRFSS is the best available source for state-specific adult asthma information. The conditions covered in the surveys may vary considerably in the accuracy and completeness with which they are reported.
Local estimates of chronic diseases are scaled in direct proportion to the base population of the county and its age distribution. No adjustments are made for other factors that may affect local prevalence (e.g., local prevalence of cigarette smokers or occupational exposures) since the health surveys that obtain such data are rarely conducted on the county level. Because the estimates do not account for geographic differences in the prevalence of chronic and acute diseases, the sum of the estimates for each of the counties in the United States may not exactly reflect the national estimate derived from the NHIS or state estimates derived from the BRFSS.
REFERENCES
Irwin, R. Guide to Local Area Populations. U.S. Bureau of the Census. Technical Paper Number 39 (1972).
National Center for Health Statistics. Raw Data from the National Health Interview Survey, United States, 2005. Calculations by the American Lung Association Research and Program Services Division using SPSS and SUDAAN software.
Centers for Disease Control and Prevention. Behavioral Risk Factor Surveillance System, 2005.
Population Estimates Branch, U.S. Census Bureau. County Resident Population Estimates, by Age, Sex, and Race: July 1, 2005.
Office of Management and Budget. Update of Statistical Areas Definitions and Guidance on Their Uses. OMB Bulletin 06-01 Corrected. Dec. 5, 2005.
