The ecological fallacy is a situation that can occur when a researcher or analyst makes an inference about an individual based on aggregate data for a group. For example, a researcher might examine the aggregate data on income for a neighborhood of a city, and discover that the average household income for the residents of that area is $30,000.
To state that the average income for residents of that area is $30,000 is true and accurate. No problem there. The ecological fallacy can occur when the researcher then states, based on this data, that people living in the area earn about $30,000. This may not be true at all, and may be an ecological fallacy.
Closer examination of the neighborhood might discover that the community is actually composed of two housing estates, one of a lower socio-economic group of residents, and one of a higher socio-economic group. In the poorer part of town, residents earn on average $10,000 while the more affluent citizens can average $50,000. When the researcher stated that individuals who live in the area earn $30,000 (the mean rate) this did not account for the fact that the average in this example is constructed of two disparate groups, and it is likely that not one person earns around $30,000.
Assumptions made about individuals based on aggregate data are vulnerable to the ecological fallacy.
This does not mean that identifying associations between aggregate figures is necessarily defective, and it doesn’t necessarily mean that any inferences drawn about associations between the characteristics of an aggregate population and the characteristics of sub-units within the population are absolutely wrong either. What it does say is that the process of aggregating or disaggregating data may conceal the variations that are not visible at the larger aggregate level, and researchers, analysts and crime mappers should be careful.