2001 Adult Asthma Data: Technical Information

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BRFSS Questionnaire

In 2001, 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 1, page xviii of BRFSS User’s Guide [PDF – 1.7 MB]). Asthma prevalence data are produced from the responses to two asthma questions on the year 2001 BRFSS Core Questionnaire.

Lifetime asthma:
6.1 Have you ever been told by a doctor, nurse, or other health professional that you had asthma? (variable name: asthma2)

Current asthma:
If the response to 6.1 was Yes, then question 6.2 was asked: Do you still have asthma? (variable name: asthnow)

Subgroup tables are based on data from the following questions:

Survey Design and Sample Weights

The survey design is described in the technical pages document, entitled, “2001 Overview” at http://www.cdc.gov/brfss/annual_data/annual_2001.htm. Sample weights are assigned, according to BRFSS methodology, to the variable _FINALWT as described in the BRFSS Weighting Formula.

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Data Analysis

Data Used:

The BRFSS 2001 Survey Data file was used to calculate estimates for all states.

Software:

Prevalence and standard error estimates were calculated using SUDAAN Release 9.0 (Research Triangle Institute, P.O. Box 12194, Research Triangle Park, NC 27709).

Data Management:

Only respondents with values of the variables as described in the table above are included in each prevalence table. Responses of “don’t know/not sure,” “refused,” or missing values were excluded.

For the question, “Do you still have asthma?” respondents included only those who answered “Yes” to question 6.1, “Did a doctor ever tell you that you had asthma?” (asthma2). Responses to question 6.2, “Do you still have asthma?” (asthnow) were subsequently recoded (assigned a value of 2 = No) if the respondent answered “No” to question 6.1. Otherwise, this variable (asthnow) was unchanged.

Ratio of Adult Self-Reported Current to Lifetime Asthma by State:

The ratio is a survey-based estimate of prevalence of asthma based on those surveyed who responded “Yes” to question 6.1, “Did a doctor ever tell you that you had asthma?” It is not a rate ratio.

Confidence Intervals:

The 95% Confidence Intervals were calculated using the following formulae:

Lower 95% Confidence Interval = % prevalence – t (se_p)
Upper 95% Confidence Interval = % prevalence + t (se_p)

Where se_p is the standard error of the prevalence percent and t equals the z distribution value of 1.96 for state level prevalence in Table 1. For other tables with smaller sample sizes due to the use of demographic subcategories, the exact value from the t distribution appropriate to the sample size for the percent prevalence was used instead of the approximate z-value of 1.96. Because of the use of the more exact t-value, 95% Confidence Intervals in the tables may differ slightly from those presented in other representations of these data.

Table Conventions:

The “US Total” line of each table excludes Puerto Rico, Guam and the Virgin Islands. “US Total,” therefore, designates an estimate for the 50 states and the District of Columbia combined.

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

Washington, D.C., is omitted from the maps. Its area is so small that the map colors and patterns were not discernible.

Ranges in all maps (both overall and for subgroups) are based on quintiles of the overall prevalence estimates from year 2000 data. These same ranges will be used in future years to facilitate year-to-year comparison of the maps.

Chart:

The chart presents the state and territory prevalence percents from Table 1 sorted from high to low. Confidence intervals are also represented graphically for each prevalence value. Overlapping confidence intervals for two states is a rough indication that state prevalence values are not significantly different from one another.

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: