Description of Methodology cont'd
Calculations of Populations-at-Risk
Presently, state (with the exception of adult asthma) and county-specific measurements of the number of persons with chronic and acute lung disease are not available. In order to assess the magnitude of lung disease 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 to project the prevalence of lung disease within the counties served by Lung Association constituents and affiliates.
Population Estimates
The U.S. Census Bureau estimated data on the total population of each county in the United States for 2002. The Census Bureau also estimated the age specific breakdown of the population by county.
Prevalence Estimates
Chronic Bronchitis, Emphysema and Pediatric Asthma. In 2002, the National Health Interview Survey (NHIS) estimated the nationwide annual prevalence of diagnosed chronic bronchitis at 9.1 million; the nationwide lifetime prevalence of emphysema was estimated at 3.1 million. The NHIS estimates the prevalence of diagnosed pediatric asthma to be over 6.0 million under age 18.
Due to the revision of the Health Interview Survey questionnaire, prevalence estimates from the 2000 State of the Air Report cannot be compared to later publications. Estimates for chronic bronchitis and emphysema can be compared to the 2001, 2002, and 2003 State of the Air Reports. However, 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 cannot be compared to previous estimates, due to another change to the National Health Interview Survey. Pediatric asthma prevalence estimates found in this report represent current asthma prevalence, not asthma attack prevalence as was depicted in the past three State of the Air Reports. Subsequently, pediatric asthma estimates will be much higher in this report than in previous ones due to the nature of the question.
Local area prevalence of chronic bronchitis, emphysema and pediatric asthma are estimated by applying age-specific national prevalence rates from the 2002 NHIS to age-specific county-level resident populations obtained from the U.S. Bureau of the Census web site. Prevalence estimates for chronic bronchitis and
emphysema are calculated for those 18-44, 45 to 64 and 65+. The prevalence estimate for pediatric asthma is calculated for those under age 18.
Adult Asthma. In 2002, the Behavioral Risk Factor Surveillance System (BRFSS) survey indicated that approximately 7.5% 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 come from the National Health Interview Survey.
The prevalence estimate for adult asthma is calculated for those 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 2002 BRFSS to age-specific countylevel resident populations obtained from the U.S. Bureau of the Census web site.
Cardiovascular Disease Estimates. All cardiovascular disease estimates were obtained from the American Heart Association: Heart Disease and Stroke Statistics – 2004 Update. According to this report, 64.4 million Americans suffer from one or more types of cardiovascular disease.
Local area prevalence of cardiovascular disease is estimated by applying age specific prevalence rates from the 2004 American Heart Association Report to age-specific county-level resident populations obtained from the U.S. Bureau of the Census web site.
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 survey.
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 acute and chronic lung disease on the national level, and the BRFSS is the best available source for adult asthma information. The conditions covered in the survey may vary considerably in the accuracy and completeness with which they are reported.
Local estimates of chronic lung 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 by the NHIS or state estimates derived by 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, 2002. Calculations by the American Lung Association Research and Scientific Affairs Division using SPSS and SUDAAN software.
Centers for Disease Control and Prevention. Behavioral Risk Factor Surveillance System, 2002.
Population Estimates Branch, U.S. Bureau of the Census. County Resident Population Estimates, by Age, Sex, and Race: July 1, 2002. American Heart Association: Heart Disease and Stroke Statistics – 2004 Update.
