Here's the Wikipedia definition:
An ecological fallacy (or ecological inference fallacy) is an error in the interpretation of statistical data in an ecological study, whereby inferences about the nature of specific individuals are based solely upon aggregate statistics collected for the group to which those individuals belong. This fallacy assumes that individual members of a group have the average characteristics of the group at large. This can also be referred to as the fallacy of division.Isn't that just wonderful?
It certainly goes an awfully long way towards explaining fat prejudice...because we all know that ALL overweight people are lazy, junk food addicts who got themselves into this fine mess all on their own and are wholly responsible for their terrible health and their shortened lifespan. That's what the media tell us and the media always tell the truth, based on infallible scientific data.