Thomas Richardson

"Why is Simpson's paradox a paradox?"

Presented to the Center for Statistics and the Social Sciences, University of Washington, 5 March 2002.

Simpson's paradox is the phenomenon whereby the direction of association present within stratified tables is reversed in the marginal table. The controversy in the 1970's over apparent gender-bias in admissions to Berkeley is a well-known instance.

Simpson's paradox is well understood from a statistical perspective. However, the traditional analysis leaves open the following questions:

1. Why do we describe this phenomenon as paradoxical?
2. When is it appropriate to stratify?

In this talk I will review work by Judea Pearl and others which attempts to answer these questions by making reference to causal concepts.