[Methodological problems in health-environment ecological studies].

Different approaches are used to study the effects of the environment on health. We restrict our discussion to observational studies at the aggregated level, the so-called ecological studies. We discuss several sources of bias for group-level studies and consider questions relating to the link between individual-level and group-level dose-effect relationship, the difference between group exposure and environmental exposure, and the influence of measurement error and variability in the exposure. Taking into consideration confounding factors in the analyses is another important item that is discussed. A final item concerns the necessity of studying the temporal direction of the effect, as well as assessing the existence of a potential threshold in the effect. As a broad conclusion, we can say that realistic quantification of uncertainty in dose-effect relationships is a delicate task that requires the systematic consideration of all sources of variability, as well as a transparent sensitivity analysis of the choices and hypotheses made during the statistical analysis.