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Description of Methodology, Cont'd...
Weighted averages allow comparisons to be drawn based on severity of air pollution. For example, Jessamine County, Kentucky, received a D because it had 9 orange days and 0 red days, earning it a weighted average of 3.0. However, York County, Maine, received an F although it only had 6 orange days, because it also had 3 red days, which signify days with more serious air pollution. York County had a weighted average of 3.5.
Note that this system differs significantly from the methodology EPA uses to determine violations of the ozone standard. EPA determines whether a county violates the standard based on the 4 th maximum daily 8-hour ozone reading each year averaged over three years. Multiple days of unhealthy air beyond the highest four in each year are not considered. By contrast, the system used in this report recognizes when a community’s air quality repeatedly results in unhealthy air throughout the three years. Consequently, some counties will receive grades of F in this report showing repeated instances of unhealthy air, while still meeting EPA’s 1997 ozone standard or the 1-hour ozone standard set in 1979.
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 level, we have employed a synthetic estimation technique originally developed by the U.S. Bureau of the Census. This method uses agespecific national estimates of reported lung disease to project the prevalence and incidence 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 2000. The Census Bureau also estimated the age specific breakdown of the population by county.
PREVALENCE ESTIMATES
Chronic Bronchitis, Emphysema and Pediatric Asthma
In 2000, the National Health Interview Survey (NHIS) estimated the nationwide annual prevalence of diagnosed chronic bronchitis at 9.4 million; the nationwide lifetime prevalence of emphysema was estimated at 3.1 million. The NHIS estimates the prevalence of diagnosed pediatric asthma to be close to 4.0 million under age 18. 2000 represents the most recent year of publication of prevalence data for the Health Interview Survey, and so was utilized to calculate county-specific prevalence. Due to the change in the Health Interview Survey questionnaire, the prevalence estimates calculated for these purposes will differ from those delineated in the 2000 State of the Air Report. However, this year’s estimates can be compared to the 2001 and 2002 State of the Air Report. Additionally, estimates for chronic bronchitis and emphysema should not be summed since they represent different types of prevalence estimates.
Local area prevalence of chronic bronchitis, emphysema and asthma are estimated by applying age-specific national prevalence rates from the 2000 NHIS to agespecific 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 2001, the Behavioral Risk Factor Surveillance System (BRFSS) survey indicated that approximately 7.2 percent of adults residing in the United States reported having asthma. The information on adult asthma obtained in the Behavioral Risk Factor Surveillance System survey cannot be compared with that from the National Health Interview Survey. Additionally, estimates for pediatric and adult asthma should not be summed since they represent different types of prevalence estimates.
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 2001 BRFSS 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 error 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 exposure) 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, 2000. Calculations by the American Lung Association Best Practices Division using SPSS and SUDAAN software.
* Centers for Disease Control and Prevention. Behavioral Risk Factor Surveillance System, 2001.
* Population Estimates Branch, U.S. Bureau of the Census. County Resident Population Estimates, by Age: July 1, 2000.
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