been exposed?" "Have we been affected?" "Will we be affected later?" They might well ask, also, "Do our management programs have any effect on the health of the public?" Table 5-1 outlines some epidemiologic research strategies that address these concerns. It shows that many types of epidemiologic studies and data can be used to determine the relation between the environment and human health. Although experimental studies of animals and laboratory studies of humans do provide some answers to these questions, epidemiologic research is essential to their resolution. Often, however, epidemiologic studies are neither available nor possible, and policy must be based on toxicologic evidence and animal studies.

Considerations of cost, urgency, and limited special expertise often require that officials rely on analyses of existing data that were gathered for other purposes. Epidemiologic studies of the classical kind involve the measurement of both the health status and the environmental exposure (or internal dose) of the persons being studied. However, such measurements cannot always be obtained. For instance, if historical exposures were not measured, the investigator may have to estimate them from other, less reliable, information. On the other hand, the exposure might be so extensive that no suitable control population remains. Individualized measures of exposure and health can also be infeasible or too expensive when a health effect occurs so infrequently that adequate study would require that a large number of exposed persons be evaluated in detail; such problems require other epidemiologic methods or the use of secondary data.

During development and implementation of public-health policy, analyses of secondary data are important at several stages. Intense study of selected small groups of people can provide useful information about risk that identifies a need for public policy. To determine the extent of potential exposure, the size and characteristics of the population exposed, or the background frequency of the health effect of interest, secondary data analyses are useful. Information from existing data systems is useful in program planning and development when data are needed to validate findings of earlier targeted research studies.

During implementation, data from existing systems can provide additional insights for public-health policy. Existing data systems tend to reflect the programmatic and regulatory structure of government programs, so the identification of useful systems (or of their absence) might help to define the most appropriate needs for assessment and availability of the public-health response. This in turn allows for midcourse changes to reduce costs, improve response, or otherwise improve on-going programs.

Data systems are a primary mechanism for evaluating the impact of a public-health policy. For instance, a public-health program might target

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