Question

The following is a classic 2x2 table:

Case Control

Exposed a b

Unexposed   c d

a. What does a/b signify?

b. What does c/d signify?

c. What is the formula, using this table, for calculating an odds ratio?

d. Based on this table, and the formula from c, what would an odds ratio of 0.8 mean? 1.0? 3.2?

Answer 1

	Case	Control
Exposed	a	b
Unexposed	c	d

a) a/b signify odds of the event(or cases) in the exposed group

b) c/d signify odds of event(or cases) in the unexposed group

C) Odds ratio= odds of cases in the exposed group / odds of cases in the unexposed group  

= (a/b) / (c/d)

= ad / bc

d) Odds ratio = 1 . there is no effect of exposure on case (occurrence of events)

Odds ratio greater than 1 signifies higher odds of cases in exposed group compared to unexposed group .

That is ,odds ratio > 1 ; increased occurrence of cases in the exposed group

Odds ratio less than 1 signifies lower  odds of cases in exposed group compared to unexposed group .

That is ,odds ratio < 1 ; decreased occurrence of cases in the exposed group

Thus Odds ratio =0.8 means odds are 0.8 times lower in cases  in exposed group compared to unexposed group. That is there is 0.8 times decreased cases in exposed group compared to unexposed group.

Odds ration = 1.0 means there is no effect of exposure on cases

Odds ratio = 3.2 means  odds are 0.8 times higher in cases in exposed group compared to unexposed group.That is there is 3.2 times increased cases in exposed group compared to unexposed group.