2002 Child Asthma Data: Technical Information
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The BRFSS survey was conducted in all 50 states, the District of Columbia and in three US territories (Guam, Puerto Rico and the Virgin Islands). In each area where interviews are conducted, respondents are randomly selected from noninstitutionalized civilian adults (18 years of age or older) living in households with a telephone (Chapter 4, page 7 of the BRFSS User's Guide [PDF - 1.7 MB]).
Child asthma prevalence data are produced from the responses to two asthma questions on the BRFSS Childhood Asthma Optional Module. An optional module is comprised of questions on specific topics that states elect to use on their questionnaire. The randomly selected adult provided responses to the child asthma questions.
The States and Territories that used the BRFSS Childhood Asthma Optional Module in 2002 are:
Alabama, Alaska, California, Delaware, Hawaii, Illinois, Indiana, Iowa, Louisiana, Massachusetts, Michigan, Mississippi, New Hampshire, New Jersey, North Carolina, North Dakota, Ohio, Oklahoma, Oregon, Rhode Island, South Carolina, South Dakota, Texas, Vermont, Wisconsin and Wyoming. The Virgin Islands were excluded from the tables because the required data elements, metropolitan area definitions and population counts were not available to create weights.
Lifetime asthma: Question number 9.01
“Earlier you said there were [fill in number] children age 17 or younger living in your household. How many of these children have ever been diagnosed with asthma?” (variable name: CASTHDX)
Current asthma: Question number 9.02
If the response to 9.01 was one or more children, then question 9.02 was asked: “Does this child/How many of these children still have asthma?” (variable name: CASTHNOW)
Survey Design and Sample Weights
The survey design is described in the technical document, entitled, “BRFSS Overview" at http://www.cdc.gov/brfss/annual_data/annual_2002.htm.
Sample weights are assigned, according to the child weighting methodology below, to the variable CHILDWT_HH.
Child Weighting Formula
CHILDWT_HH = (_FINALWT/(POSTSTRAT*NAD))*POSTSTRAT2
All of the components in _FINALWT, which are not specific to adults were included in the creation of the weights for children. Converting _FINALWT into a child weight involved the following steps:
- Dividing out the number of adults (NAD) in the household.
- Dividing out the post-stratification coefficient (POSTSTRAT) for the adult population and replacing it with the post-stratification coefficient for the state’s child population (POSTSTRAT2).
CHILDWT_HH is the child household weight assigned to each respondent with children in the household. If the number of children (CHILDREN) in the household was zero or unknown, CHILDWT_HH has a value of zero. This weight does not factor in total number of children in the household (CHILDREN), which makes it applicable to only variables that include information for all children in the household.
_FINALWT is the weight assigned to each adult respondent.
POSTSTRAT is the adult post-stratification coefficient.
NAD is the number of adults in the respondent’s household.
POSTSTRAT2 is the child post-stratification coefficient. It is the census child population count by state and urban/rural status divided by the weighted number of children sampled by state and urban/rural status.
2002 child post-stratification data sources:
- Urban/rural status was defined according to the Office of Management and Budget, June 30, 1999 metropolitan area definitions. County codes used to identify urban/rural status were obtained from the U.S. Census Bureau’s website at: http://www.census.gov/population/estimates/metro-city/99mfips.txt and http://www.census.gov/population/estimates/metro-city/99nfips.txt.
- Child population counts by state and urban/rural status were obtained from U.S. Census Bureau American Fact Finder - Table P29: Relationship by age for the population under 18 years, in metro area or not in metro area and Data Set: Census 2000 Summary File 1 (SF 1).
- Missing county codes were imputed using a database provided by the Behavioral Surveillance Branch, Division of Adult and Community Health, National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention.
Note: This procedure was developed by Research Triangle Institute under a contract with CDC.
The BRFSS 2002 Survey Data file was used to calculate estimates for 2002.
Prevalence and standard error estimates were calculated using SUDAAN Release 9.0.0 (Research Triangle Institute, P.O. Box 12194, Research Triangle Park, NC 27709).
Responses of “don’t know/not sure,” “refused,” or missing values were excluded.
Confidence Intervals and Prevalence Numbers:
The 95% Confidence Intervals were calculated using the following formulas:
Lower 95% Confidence Interval = % prevalence – t (sep)
Upper 95% Confidence Interval = % prevalence + t (sep)
Where sep is the standard error of the prevalence percent and t equals the z distribution. The t-value of 1.96 was used to calculate confidence intervals.
In 2002, Illinois used a disproportionate sampling method with clusters of size 1 and a split-sample design. The two asthma questions were only asked of half their total sample. For respondents in Illinois, prevalence numbers and 95% confidence interval numbers were calculated differently from other states in order to properly accommodate the split-sample design used in that state. Because of this difference the Illinois counts were not included in the total line for the 25 states.
Calculation for 2002 Illinois current prevalence (number):
207,709 (prevalence number) = 6.4 (prevalence percent) * 3,245,451 (Census 2000, Illinois age 0-17 population)
Calculation for 2002 Illinois current confidence interval (number):
162,273 (lower 95% confidence interval number) = 5.0 (lower 95% confidence interval percent) * 3,245,451 (Census 2000, Illinois age 0-17 population)
249,900 (upper 95% confidence interval number) = 7.7 (upper 95% confidence interval percent) * 3,245,451 (Census 2000, Illinois age 0-17 population)
Calculation for 2002 Illinois lifetime prevalence (number):
305,072 (prevalence number) = 9.4 (prevalence percent) * 3,245,451 (Census 2000, Illinois age 0-17 population)
Calculation for 2002 Illinois lifetime confidence interval (number):
253,145 (lower 95% confidence interval number) = 7.8 (lower 95% confidence interval percent) * 3,245,451 (Census 2000, Illinois age 0-17 population)
357,000 (upper 95% confidence interval number) = 11.0 (upper 95% confidence interval percent) * 3,245,451 (Census 2000, Illinois age 0-17 population)
In 2002, New Jersey’s standard error is high compared to other states due to data collection issues and over-sampling.
In the tables, states are listed in Federal Information Processing Standard (FIPS) code order, except for Illinois in 2002.