the ratio of the cross-products.
•
Odds ratio = ad
bc
•
Odds ratio = 6 x 7
= 42
= 3.5
4 x 3
12
Cases
Controls
Exposed
6
3
Nonexposed
4
7
Total
10
10
Gordis, Epidemiology, 5
th
edition, Page 224
Examples of calculating Odds Ratios in Case-
control studies

•
Matched-Pairs Case-Control study:
•
Controls are selected by matching each one to a case
according to variables that are known to be related to
disease risk, such as sex, age, or race (individual
matching or matched pairs). The results are then
analyzed in terms of case-control pairs rather than for
individual subjects.
•
If exposure is dichotomous (a person is either exposed
or not exposed), four types of case-control pairs or
combinations are possible
.
Examples of calculating Odds Ratios in Case-
control studies

Concordant pairs
(pairs
with the same exposure
experience)
1.
Pairs in which both the
case and the control
were exposed
2.
Pairs in which neither the
case nor the control was
exposed
Discordant pairs
(pairs
with different exposure
experience)
3.
Pairs in which the case
was exposed but the
control was not
4.
Pairs in which the control
was exposed and the
case was not
Gordis, Epidemiology, 5
th
edition, Page 225
Examples of calculating Odds Ratios in Case-
control studies

Gordis, Epidemiology, 5
th
edition, Page 225
Examples of calculating Odds Ratios in Case-
control studies
Exposed
Not Exposed
Exposed
a
b
Not Exposed
c
d
Control
Case

Examples of calculating Odds Ratios in
Case-control studies
•
Calculation of the odds ratio in such a matched-
pair study is based on the
discordant pairs
only
(
b
and
c
). The concordant pairs are ignored,
because they do not contribute to our knowledge
of how cases and controls differ in regard to past
history of exposure.
•
The
odds ratio for matched pairs
is therefore the
ratio of the discordant pairs. The odds ratio for
the preceding 2 × 2 table is as follows: Odds ratio
(matched pairs) = b/c

Examples of calculating Odds Ratios in
Case-control studies

Gordis, Epidemiology, 5
th
edition, Page 226
Examples of calculating Odds Ratios in Case-
control studies
Exposed
Not Exposed
Exposed
2
4
Not Exposed
1
3
Control
Case
•
The concordant pairs are ignored in the analysis of matched
pairs. There are four pairs in which the case was exposed and
the control was not and one pair in which the control was
exposed and the case was not.
•
Hence, the odds ratio for matched pairs is as follows:
Odds ratio = b/c = 4/1 = 4

Conclusion
•
This chapter has introduced the concepts of
absolute risk, relative risk, and odds ratio.
•
In Chapter 12 we will discuss another
important aspect of risk: the attributable risk.