tion must often be considered in relation to the population at risk and intensity of the exposure. For example, an RR of 4 that affects a small population may have a much smaller public-health impact than does an RR of 1.2 that affects much larger numbers. Epidemiologists are sometimes concerned with attributable risk, which is a measure of the rate of disease above the background rate that can be attributed to exposure. This is more difficult to detect, study, and estimate in environmental epidemiology because it is difficult to determine a baseline rate. Problems with using strength of association as the principal criterion for causality include the fact that misclassification and other biases can profoundly change the strength of association.
Specificity suggests that the suspected causal agent induces a single disease. While this may apply to a few associations between exposure and disease (e.g., vinyl chloride and angiosarcoma of the liver), single diseases (e.g., lung cancer) can have many causes, and single agents can cause many effects (e.g., lead at high-enough levels can cause increased blood pressure, neurologic symptoms, reproductive effects, and kidney damage). Specificity can be diminished by inappropriate or inaccurate grouping of diseases in a way that obscures a real effect (e.g., grouping some rare forms of cancer with other cancers).
The observed relation between exposure and disease is seen rather regularly in independently conducted studies; the value of consistency is enhanced if the studies are of different types and in different populations. For example, a study of the association between lung cancer and passive smoking may produce an RR of only 2.0 or less, but this elevated risk has now been reported in over 30 studies carried out in 6 countries (NRC, 1986). Because of the variety in study protocols and populations, claims of bias in all the studies have little credibility. Studies not having statistically significant results can be combined with similar studies, as long as they all use sound methods. Studies that meet the standards for good epidemiologic practice can be grouped for meta-analysis, which allows for statistical pooling of different studies.
The exposure should precede the development of symptoms or diseases of interest by an appropriate interval. The time between exposure