2011 Child Asthma Data: Technical Information

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Behavioral Risk Factor Surveillance System (BRFSS)

In 2011, the BRFSS survey was conducted in all 50 states, the District of Columbia and in one US territory (Puerto Rico).

In each area where interviews were conducted, respondents were randomly selected from non-institutionalized civilian adults (18 years of age or older) living in households with a telephone (Chapter 4, page 7 of the BRFSS BRFSS User's Guide [PDF - 1.7 MB]).

Child asthma prevalence data are produced from the responses to questions on BRFSS Random Child Selection and Childhood Asthma optional modules. A randomly selected adult provided responses to the child asthma questions. If the adult reported that one or more children aged 17 years or younger lived in the household, then one of the children was randomly selected as the “Xth” child. Questions on the BRFSS Random Child Selection and Childhood Asthma optional modules were asked of the “Xth” child.

Sixteen states and one territory used both the BRFSS Random Child Selection and Childhood Asthma optional modules in 2011. These states and territories were

Arizona, Connecticut, Hawaii, Michigan, Montana, , New Jersey, New Mexico, Oregon, Pennsylvania, Tennessee, Texas, Utah, Vermont, Washington, West Virginia, Wisconsin, and Puerto Rico.

The following two asthma questions pertinent to prevalence estimation were asked on the BRFSS Childhood Asthma Prevalence optional module.

Lifetime asthma: Question number 33.1
“Has a doctor, nurse or other health professional ever said that the child has asthma?” (Variable name: CASTHDX2 [Yes = 1 / No = 2])

Current asthma: Question number 33.2
“Does the child still have asthma?” (Variable name: CASTHNO2 [Yes = 1 / No = 2])

Subgroup tables are based on data from the following questions of Random Child Selection optional module.

Survey Questions, Variable Names and Codes Used in Subgroup Tables

BRFSS Sampling and Weighting Changes

The 2011 BRFSS data collection changed from a landline sample survey to a dual-mode survey, landline and cellphone. The 2011 BRFSS data reflects a change in weighting methodology (raking) and the addition of cell phone only respondents. The aggregate BRFSS combined landline and cell phone dataset is built from the landline and cell phone data for 2011 and includes data for 50 states, the District of Columbia and Puerto Rico. Guam and the Virgin Islands were not included in the initial data release because current population estimates were not available. These territories will be included after the 2010 Census estimates have been released. More information about the changes to the 2011 BRFSS can be found at the Web location: http://www.cdc.gov/surveillancepractice/reports/brfss/brfss.html.

For data analysis, the 2011 BRFSS data should be considered a baseline year and are not directly comparable to previous years of data because of the changes in weighting methodology and the addition of the cell phone only respondents . Please see the BRFSS Frequently Asked Questions document for additional information: http://www.cdc.gov/surveillancepractice/reports/brfss/brfss_faqs.html

Survey Design and Sample Weights

The survey design and the child-weighting methods are described in the technical document, “OVERVIEW: BRFSS 2011”, which can be found at the Web location: http://www.cdc.gov/brfss/annual_data/annual_2011.htm.

Data Analysis

Data Used:

The BRFSS 2011 survey dataset was used to calculate prevalence estimates for all states that had the Random Child Selection & Childhood Asthma optional modules included in the common questionnaire.

Software:

Prevalence and standard error estimates were calculated using SAS callable SUDAAN Release 11.1 (Research Triangle Institute, Research Triangle Park, NC).

Data Management:

Responses of “don’t know/not sure,” and “refused” were re-coded as missing values. Children older than 18 years were excluded from further analysis.

Confidence Intervals and Prevalence Numbers:

The 95% confidence intervals were calculated using the following formulas:

Lower 95% confidence interval = % prevalence – t_v (se_p)
Upper 95% confidence interval = % prevalence + t_v (se_p)

Where se_p is the standard error of the prevalence percent and t_v represents the student’s t distribution with v degrees of freedom.

Small Sample Size and Failure of Distributional Assumptions

When sample sizes for a particular state or territory was smaller than 50, the calculated standard error of the prevalence estimate might have been large relative to the point estimate of the prevalence value, which caused a wide 95% confidence interval. This sometimes caused the lower limit of the 95% confidence interval to be negative. In which case, the lower limit of the interval was set to zero.

In situations where the normal distribution approximation to the binomial distribution did not hold, values for the standard error and the 95% confidence interval of estimates were not provided, because these estimates are not reliable.

Table Conventions:

In the tables, states and territories are listed in Federal Information Processing Standard (FIPS) code order.