Epidemiology/Introduction

Block 1: Epidemiology

EPI.4-1: THE ODDS RATIO

SESSION OBJECTIVE

At the end of this session you should be able to understand the Odds Ratio as a measure of effect and what it estimates. You should also be able to calculate it from data.

LEARNING ACTIVITIES:

Note that the Odds Ratio has no easily usable counterpart as a measure of occurrence.

It is used in a case-control design study where one has to work backwards from a given number of cases and controls to the probability of exposure, rather than directly measure the probability of disease (measure of occurrence).

Remember the definition:

	D+	D-
E+	a	c	M₁
E-	b	d	M₀
	N₁	N₁	T

The odds of exposure in the diseased (OED+) is given by

Similarly, the odds of exposure in the non-diseased (OED-) is

Hence