As my first introduction to the ecological fallacy, I thought it did a good job concisely stating the fallacy, with good examples to illustrate it (both intuitive and non-intuitive).

The next question that would inevitably come up is: how do you know? I'm guessing there isn't a way short of looking at the data for individuals. It would probably be safe to always assume group data does not imply individual data.

And, of course, this is another way that people can use statistics to lie to you. I would not be surprised at all to find people intentionally using this fallacy to their benefit.