Estimating health conditions for small areas: asthma symptom prevalence for state legislative districts.

Asthma is a national and state health concern that has generated much research and policy activity. In California, an estimated 3.9 million children and adults, 11.9 percent of the state's population, reported that they had been diagnosed with asthma at some point in their lives (Meng et al. 2003). About 3 million children and adults who had ever been diagnosed with asthma also experienced asthma symptoms, such as wheezing, breathlessness, chest tightness, and nighttime or early morning coughing (Sears 1997), at least once during the previous 12 months (Mendez-Luck, Yu, Meng, Chia, Jhawar et al. 2004). The 2003–2004 California legislative session considered 28 bills focusing explicitly on asthma risk factors, seven on asthma treatment, and five that used asthma as a rationale for other action. Thirteen bills passed and were signed into law. The legislative activity related to asthma leads many elected representatives to want to understand the salience of the problem and consider the data for their jurisdictions. California also has a number of state programs and agencies that are responsible for tracking asthma and related risk factors; they also require local area data for their planning and policy work. The California Department of Health Services published a strategic plan for asthma that outlined research, education, treatment, prevention, and policy goals for the coming years. The California Asthma Public Health Initiative (see http://www.dhs.ca.gov/cdic/caphi/) funded 20 local community agencies throughout the state for programs to reduce asthma morbidity for school-aged children. It is reasonable that asthma symptom rates would vary across locales, given variations in access to care, quality of care and the triggers of asthma exacerbation in different regions (National Asthma Education and Prevention Program 2003). However, local-level information on this ambulatory-sensitive chronic disease is virtually nonexistent. Most national surveys do not typically have identifiers for smaller geographic areas such as counties. Even the California Health Interview Survey (CHIS), the largest state health survey in the country—designed to make county-level estimates of health status and health services utilization—is limited in its power to make subcounty estimates. California policy makers and advocates continually request data on asthma and other health issues for policy activities. However, California's Assembly, Senate, or Congressional districts do not follow any other administrative boundaries, making it difficult to assess the health of legislators' constituencies. District-specific information on the burden of disease—in this case, asthma—is especially important for establishing the need for and impact of policies. In response to ongoing requests for such data, we conducted a study to estimate the prevalence of various ambulatory care-sensitive chronic diseases, including asthma symptoms, for California's Assembly, Senate, and Congressional districts. The purpose of the study was to develop a small-area estimation (SAE) methodology to create such estimates and to produce a series of policy briefs for a broad audience of legislators and advocates. The policy briefs can be found at http://www.healthpolicy.ucla.edu/pubs/pubList.asp?topicID=12&subTopicID=42. The primary purpose of this article is to describe the SAE method we used to determine asthma symptom prevalence among California residents. We defined “asthma symptom prevalence” as ever having had a prior diagnosis of asthma and having asthma symptoms in the previous 12 months. The secondary purpose is to illustrate the utility of SAE methods for examining health issues at local levels, using asthma symptom prevalence at the Assembly district level as an example. Rao (2003) and Pfeffermann (2002) provide comprehensive reviews of the methodologies and applications of SAE in health-related research. Few studies have applied SAE to asthma symptom prevalence (Gottlieb et al. 1995; Salmond et al. 1999; Cesaroni et al. 2003). We used SAE methods to identify asthma patterns throughout the state for the purposes of estimating asthma symptom prevalence among the state's residents. We specifically sought to provide policy audiences with data that aided their efforts to identify the scope of asthma prevalence in their areas. We improved upon a methodology previously developed to study health insurance coverage in California (Yu et al. 2007). The method is similar to the one independently developed by Twigg et al. (2000), with key differences that are elaborated elsewhere (Yu et al. 2007).