Epidemiology/Introduction

Block 1: Epidemiology

EPI4-4: THE ODDS RATIO:LINK WITH THE OTHER MEASURES OF EFFECT

SESSION OBJECTIVE

At the end of this session you should understand the link between the Odds Ratio and other measures of effect - the prevalence ratio, the incidence proportion ratio and the incidence rate ratio.

LEARNING ACTIVITIES:

The relationship between the Odds Ratio and the Prevalence Ratio or Incidence Proportion Ration is simpler to understand.

	D+	D-
E+	a	c	M₁
E-	b	d	M₀
	N₁	N₁	T

If the data are based on a prevalence or an incidence study in a closed cohort,the OR can also be worked out a third way.

The prevalence ratio or the incidence proportion ratio of disease in the exposed to that in the unexposed is (a/M₁) / (b/M₀),

which is equal to (a/a + c)/(b/b + d).

If the prevalence (a+b)/T is small (usually less than 10%) , it can be assumed that a and b are small in relation to c and d and then (schematically)

(a + c) ~ c and (b + d) ~ d,

and the above prevalence ratio is approximately (~) ad/bc which is the Odds Ratio.

Example:

	D+	D-
E+	53	53	106
E-	43	85	128
	96	138	234

If the data come from either type of study rather than a case control study the Prevalence Ratio or the Incidence Proportion Ratio would be 53 x 128/106 x 43 = 1.49, which is close to the OR of 1.89.

General Introduction to Occupational Health: Occupational Hygiene, Epidemiology & Biostatistics by Prof Jonny Myers is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 2.5 South Africa License.