Title: Ecological inference and the ecological fallacy
Author: David A. Freedman 
Date: March 1999
Pub: PDF
Url: http://www.stat.berkeley.edu/~census/549.pdf
Abstract:
This paper reviews several methods for making ecological
inferences, that is, inferring the behavior of individuals
from aggregate data.  Also considered is the ecological
fallacy, which is the idea that relationships observed for 
groups necessarily hold for individuals.