# 2004 Adult Asthma Data: Technical Information

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## BRFSS Questionnaire

In 2004, the BRFSS survey was conducted in all 50 states, the District of Columbia and in two US territories (Puerto Rico and the Virgin Islands). The data for Hawaii and Guam was not available. 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 the BRFSS User's Guide [PDF - 1.7 MB]). Asthma prevalence data are produced from the responses to two asthma questions on the year 2004 BRFSS Core Questionnaire.

**Lifetime asthma: Question number 9.01**

“Have you ever been told by a doctor, nurse, or other health professional that you had asthma?” (variable name: ASTHMA2)

The calculated variable _LTASTHMA (based on question 9.01) from the public use file was used when creating the tables.

**Current asthma: Question number 9.02**

If the response to 9.01 was Yes, then question 9.02 was asked: “Do you still have asthma?” (variable name: ASTHNOW)\

The calculated variable _CASTHMA (based on questions 9.01 and 9.02) from the public use file was used when creating the tables.

Subgroup tables are based on data from the following questions:

Survey Question | Variable name | Codes used |
---|---|---|

13.16 Indicate sex of respondent | sex | 1 = male 2 = female |

13.1 What is your age? | age | Grouped into: 18-24, 25-34, 35-44, 45-54, 55-64, 65+ |

13.2 Are you Hispanic or Latino? | hispanc2 | 1 = yes 2 = no |

13.3 Which one or more of the following would you say is your race? | _mrace | 1 = white 2 = black or African American 3 = Asian 4 = Native Hawaiian, Pacific Islander 5 = American Indian, Native Alaskan 6 = other race 7 = multiracial |

Responses to 13.2 and 13.3 are used to define race/ethnicity | _racegr2 | 1 = white, non-Hispanic 2 = black, non-Hispanic 3 = other, non-Hispanic 4 = multiracial, non-Hispanic 5 = Hispanic |

13.7 What is the highest grade or year of school completed? | educa | 1,2,3 = <HS graduate 4 = HS graduate 5 = some college 6 = college graduate |

13.9 Is your annual household income from all sources: a. Less than $25,000? etc. | income2 | 1,2 = <$15,000 3,4 = $15,000-$24,999 5,6 = $25,000-$49,999 7 = $50,000-$74,999 8 = $75,000+ |

## Survey Design and Sample Weights

The survey design is described in the technical pages document, entitled, "2004 Overview" at http://www.cdc.gov/brfss/annual_data/annual_2004.htm. Sample weights are assigned, according to BRFSS methodology, to the variable _FINALWT as described in the BRFSS Weighting Formula.

## Data Analysis

##### Data Used:

The BRFSS 2004 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.

##### 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 9.01, “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:

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
- Page last updated: April 27, 2009
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