Estimators for contingency tables and their properties and assumptions.

Estimator Purpose
Proportion difference Estimate the difference of proportions (also known as Risk Difference).
Proportion ratio Estimate the ratio of proportions (also known as Risk Ratio).
Odds ratio Estimate the odds ratio.

Values of the odds ratio range from zero to infinity. Values further from 1 in either direction represent a stronger association. The inverse value of an odds ratio represents the same degree of association in opposite directions.

The odds-ratio does not change value when the orientation of a contingency table is reversed so that rows become columns and columns become rows. Therefore it is useful in situations where both variables are random.