Population-based surveys and their role in public health1

Timely and accurate health information is needed to assess the population’s health status and to determine current and future needs for healthcare services and resources. In particular, detailed information on health and healthcare characteristics of the U.S. population is extremely important to those responsible for the planning, implementation, and assessment of public health programs, as well as the formulation of public health policy. Population-based sample surveys are often used to obtain this needed information. By definition, a population-based sample survey is defined as a study involving a subset (or sample) of individuals selected from a larger population.1 Sample surveys can be based on either probability samples or nonprobability samples. In a probability sample survey, each member of the surveyed population of interest has a known, nonzero probability of selection. Equally important, people are sampled according to a method of random selection. Because of these features, probability samples produce results that can be generalized to the entire sampled population and permit unbiased estimates of characteristics of the population. In addition, the error associated with studying a sample rather than the total population can be estimated. In contrast, surveys based on a nonprobability sample do not have these features and are often referred to as convenience or quota samples. This commentary discusses probability sample surveys. The Centers for Disease Control and Prevention (CDC) conducts a variety of large, population-based, sample surveys that provide critical information about the prevalence and distribution of diseases and disease risk factors; disease etiology; health-related knowledge, attitudes, and beliefs; healthcare utilization; health behaviors; and the implementation of disease prevention and health promotion activities. Surveys also produce important data used in various types of epidemiologic studies. Evidence of the role of probability sample health surveys and how their results have impacted our society is provided by a few key examples. With its direct physical and biochemical measurements, the National Health and Nutrition Examination Survey (NHANES) provides data to assess the health and nutritional status of the U.S. population; monitor the prevalence of major diseases and risk factors; and establish reference standards for child growth and development, blood pressure, cholesterol, and various other biochemical measurements from blood and urine samples. For example, in 1996, the U.S. Food and Drug Administration mandated the fortification of grain and cereal products with folic acid, a vitamin shown to prevent birth defects such as spina bifida and other neural tube defects. Over time, NHANES surveys have measured serum and red blood–cell folate concentrations for women of childbearing age. Comparing results of the 1988–1994 NHANES to the 1999 survey shows substantial increases in folate concentrations among women of childbearing age.2 NHANES surveys have also been useful in monitoring the public health problem of elevated blood-lead levels, which cause a variety of adverse health effects, especially in cognitive development and behavioral problems among young children. The second NHANES (1976–1980) documented that 88% of young children in the United States had elevated blood-lead levels.3 NHANES also showed a decline in blood-lead levels over the survey period that coincided with the removal of lead from gasoline, food containers, plumbing, residential paint, and other consumer products. The survey data documented the success of these primary and secondary prevention efforts. Phase 2 of the third NHANES (1991–1994) showed that by the early 1990s the prevalence of elevated blood-lead levels in young children had plummeted to 4.4%—a 95% reduction from the earlier survey.4 Monitoring personal behaviors associated with health is another important function of surveys. The CDC’s Behavioral Risk Factor Surveillance System survey, for example, is a large telephone survey that includes questions regarding nutritional intake, alcohol and tobacco use, exercise, sexual practices, and healthcare utilization. One analysis from this survey found that from 1991 to 1995, prevalence of heavy alcohol use among pregnant women, at levels that have been shown to cause birth defects and developmental disabilities in the unborn child, had increased substantially.5 This finding boosted interest in public health interventions to reduce alcohol use among pregnant women. Determining personal knowledge, attitudes, and beFrom the National Center for Health Statistics, Centers for Disease Control and Prevention, Hyattsville, Maryland Address correspondence to: Trena M. Ezzati-Rice, MS, 6525 Belcrest Road, Room 915, Hyattsville, MD. E-mail: tme1@cdc.gov. Address reprint requests to: Centers for Disease Control and Prevention, National Immunization Program Resource Center, 1600 Clifton Road NE, Mailstop E-34, Atlanta, Georgia 30333. Fax: (404) 639-8828.