Methodology: Estimated Prevalence and Incidence of Lung Disease
Presently, county-specific measurements of the number of persons with chronic lung disease are not available. In order to assess the magnitude of lung disease at the county levels, this report utilizes a synthetic estimation technique originally developed by the U.S. Census Bureau. This method uses age-specific national or state estimates of diagnosed lung disease to project the prevalence of chronic lung disease and sex-specific age-adjusted state estimates to project the incidence of lung cancer for each county within the United States.
COPD and Adult and Pediatric Asthma. The Behavioral Risk Factor Surveillance Survey questionnaire asks respondents "Has a doctor, nurse, or other health professional ever told you that you had any of the following?" followed by a series of conditions. Prevalence rates in this report are based on responses to the conditions:
- "(COPD) chronic obstructive pulmonary disease, emphysema or chronic bronchitis?"
- "Asthma?" and "Do you still have asthma?"
In states fielding the optional child asthma module, adult respondents with one or more children in the household answer the same two asthma questions for the child, if only one, or a randomly selected child, if two or more are reported.
In 2016, the Behavioral Risk Factor Surveillance System (BRFSS) survey found that approximately 22.0 million (8.9 percent) of adults residing in the United States and 8.1 percent of children from thirty-one states reported currently having asthma. About 16.2 million adults (6.5 percent) reported ever being diagnosed with COPD.
Local area prevalence of adult asthma and COPD is estimated by applying age-specific state prevalence rates from the 2016 BRFSS to age-specific county-level resident populations obtained from the U.S. Census Bureau web site. Local area prevalence of pediatric asthma is estimated by applying the most recent state prevalence rates, or if none are available, the national rate from the BRFSS to pediatric county-level resident populations obtained from the U.S. Census Bureau web site. Thereafter, the age-specific prevalence estimates for each county within a state are summed to determine its overall pediatric and adult prevalence.
The prevalence estimates for pediatric and adult asthma are calculated for those under 18 years of age and 18 years of age or older, respectively. Pediatric asthma data from the 2016 BRFSS was available for thirty-one states; from the 2015 BRFSS for Washington D.C., Louisiana, New Hampshire, and Texas; from the 2014 BRFSS for Alabama, Maryland, North Carolina, Tennessee, and West Virginia; from the 2012 BRFSS for North Dakota and Wyoming; from the 2011 BRFSS for Iowa; and national data was used for the eight states (Alaska, Arkansas, Colorado, Delaware, Idaho, South Carolina, South Dakota, and Virginia) that had no data available. Data from earlier years was not used due to changes in the 2011 survey methodology.
Asthma and COPD estimates should not be compared to those from 2012 or earlier due to changes in the survey methodology and switch to a different question format, respectively.
Lung Cancer. State- and gender-specific lung cancer incidence rates for 2014 were obtained from StateCancerProfiles.gov, a system that provides access to statistics from both the NCI’s Surveillance, Epidemiology and End Results (SEER) program and the CDC’s National Program of Cancer Registries.
Local area incidence of lung cancer is estimated by applying 2014 age-adjusted and sex-specific incidence rates to 2016 county populations obtained from the U.S. Census Bureau. Thereafter, the incidence estimates for each county within a state are summed to determine overall incidence.
Limitations of Estimates
Since the statistics presented by 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 BRFSS is the best available source for asthma and COPD information on the state level. The conditions covered in the surveys 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 or state estimates derived by the BRFSS.
- Irwin, R. Guide to Local Area Populations U.S. Bureau of the Census Technical Paper Number 39 (1972).
- Centers for Disease Control and Prevention. National Center for Health Statistics. Behavioral Risk Factor Surveillance System, 2016. Calculations by the American Lung Association Research and Program Services Division using SPSS software.
- StateCancerProfile.gov, 2017. Cancer Incidence by State and Gender, 2014.
- Population Estimates Branch, U.S. Census Bureau. Annual Estimates of the Resident Population by Selected Age Groups and Sex for Counties: April 1, 2010 to July 1, 2016.