Ecological studies lack the ability to control for the effects of confounding factors. The findings of a linear relationship between average exposure and disease frequency in ecological studies do not imply that such a linear relationship will be present at the individual levels. This is known as the ´ecological fallacy´. Despite these limitations, ecological studies may be the best approach to studying exposures that are easier to measure at the group rather than the individual level because most ecological studies make use of routinely collected data. They are also useful for monitoring the effectiveness of population interventions such as vaccination programs, health education campaigns and mass screening programs. Thus, ecological studies are useful epidemiologic tools for public health surveillance if we know their limitations and interpret their results carefully. Ecological studies often help to generate hypotheses, although they rarely provide a strong test of a causal hypothesis.