What is an example of ecological fallacy in epidemiology?

An example of ecological fallacy in epidemiology is the relationship between fat consumption and breast cancer.

Studies at the population level have found that there is a correlation between higher rates of fat consumption and higher rates of breast cancer. These findings though have been misinterpreted and some people may think that individuals who consume fat are more likely to develop breast cancer. However, this is not true. A correlation at group level doesn’t necessarily mean that there is also a correlation at individual level. It also doesn’t prove a causal relationship between a high fat diet and breast cancer. Drawing conclusions about individuals based only on analyses of group data is an ecological fallacy.