2002 Child Asthma Data: Technical Information

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BRFSS Questionnaire

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 Cdc-pdf[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)

Data Analysis

Data Used:

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).

Data Management:

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.

Illinois 2002

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)

Standard Errors:

In 2002, New Jersey’s standard error is high compared to other states due to data collection issues and over-sampling.

Table Conventions:

In the tables, states are listed in Federal Information Processing Standard (FIPS) code order, except for Illinois in 2002.

Small Sample Size

When sample sizes in cells are less than 50, as they are in some of the subgroup tables, the standard error may be large relative to the prevalence value, leading to a wide 95% confidence interval. Caution should be used in interpreting such “imprecise” estimates of prevalence.

Also, when one or more of the following situations occurred, the indicated solutions were applied:

Possible Situations and the Solutions
Situation Solution
1. The normal distribution approximation to the binomial distribution did not apply. 1. Values for the standard error and the 95% confidence interval were not provided.
2. If the normal distribution approximation does apply, it is appropriate to use values of the t-distribution to compute confidence intervals. 2. Two-sided 95% confidence interval values from the t-distribution with (n-1) degrees of freedom (where n is the number of observations for the subgroup) were used.
3. The lower limit of the 95% confidence interval was negative. 3. The lower limit was set to zero.


Page last reviewed: April 24, 2009