Ecologic studies examine relationships between groups of individuals with exposure to a putative risk factor and an outcome. Exposures are measured at the population, community or group level rather than at the individual level. Ecologic studies can provide information about an association, however, they are prone to bias: the ecological fallacy. The ecological fallacy holds that relationships observed for groups necessarily hold for individuals (e.g., if countries with more dietary fat have higher rates of breast cancer, then women who eat fatty foods must be more likely to get breast cancer). These inferences may be correct, but are only weakly supported by the aggregate data.