Commentary: A niche for ecologic studies in environmental epidemiology.

The inherent limitations of ecologic studies as the basis for assessing causal relationships are well-known, starting with the “ecologic fallacy”—the potential lack of correspondence between associations identified in aggregate population groups and on the individual level. Ecologic studies are often labeled as “hypothesis-generating” studies, a questionable phrase if taken literally: If the study really generated the hypothesis, then why were the data assembled to conduct the study in the first place? The term is intended to deflect criticism, asking the reader to set the bar lower and expect less because it is just an ecologic study. A more honest label would be “susceptible to erroneous or misleading results. ” Ecologic studies often serve as a useful step in the evolution of research, providing preliminary information that requires more definitive research designs to make progress. The hypothesis exists, perhaps based only on intuition or speculation, and ecologic studies provide an efficient, preliminary, imperfect way to address it. The key question (as for any type of study) is to know when the approach is useful and when it is not, and there are some simple determinants of the contribution that can be expected from ecologic studies. First, the baseline level of knowledge against which the ecologic study is being measured has to be sufficiently limited for the contribution to be meaningful. The timeand cost-efficiency of using existing data for an ecologic study, relative to studies of individuals, means that some topics that are not deserving of more demanding research designs will be worthy of ecologic study. Prior to the report by Wheeler and colleagues on radon and skin cancer, the published literature consisted of a single, less sophisticated ecologic study and theoretical calculations of possible risk, setting the stage for a well-designed, carefully analyzed ecologic study to provide a substantial increment in knowledge. Nonmelanoma skin cancer has been studied far less extensively than other types of cancer, largely because it is rarely included in cancer registries. Therefore, the availability of comprehensive, high-quality data on nonmelanoma skin cancer across a large geographic area in which baseline risk is elevated provides a unique opportunity that would not be applicable to other cancer types that have been more widely registered and subjected to more intensive study. In contrast, such an approach would add little to our understanding of radon and lung cancer, for example, which has been rigorously examined in many individual-level studies. Second, the exposure needs to have marked variation across geographic units, sufficiently dramatic to overcome the inevitable blurring that results from aggregation. The ideal, of course, is for all the variation in exposure to be between units and none within, at which point the ecologic approach is as effective in assigning exposure as a study of individuals would be. To the extent that location is a direct determinant of exposure, in the case of radon arising as a function of local geology, the balance of withinand between-area variation is likely to be more favorable. When exposure variation arises