3 Algebraic geometry of 2 × 2 contingency tables

Contingency tables represent the joint distribution of categorical variables. In this chapter we use modern algebraic geometry to update the geometric representation of 2 × 2 contingency tables first explored in (Fienberg 1968) and (Fienberg and Gilbert 1970). Then we use this geometry for a series of new ends including various characterizations of the joint distribution in terms of combinations of margins, conditionals, and odds ratios. We also consider incomplete characterisations of the joint distribution and the link to latent class models and to the phenomenon known as Simpson’s paradox. Many of the ideas explored here generalise rather naturally to I × J and higher-way tables. We end with a brief discussion of generalisations and open problems.