2x2 Table / Contingency Table

Definition: A 2x2 or contingency table categorizes study subjects for many purposes.  In outbreak investigations and other studies, the categorization is often based on disease and exposure status. Commonly we classify study participants as follows:

A = number of people with both the disease and exposure

B = number of people with the exposure but not the disease

C = number of people with the disease but not the exposure

D = number of people with neither the disease nor the exposure

These assigned variables can then be applied to formulas for measures of association. Such formulas are used to calculate a study's relative risk and odds ratio.

Layout of a 2x2 table:

 

Disease
Yes
Disease
No
Total
Exposure
Yes
A B A+B
Exposure
No
C D C+D

 

A+C B+D A+B+C+D

 

Example: To calculate the association between flea bites and plague, a 2x2 table can be used to categorize people based on disease and exposure status. Let's say there are 100 people in a study, 50 with plague and 50 without. Of those with plague, 40 reported having a flea bite, leaving 10 who did not have a flea bite. Of those who did not have plague, 20 had a flea bite and 30 did not. Using this information, the boxes of the 2x2 table can be filled in as seen below.

 

Plague
Yes
Plague
No
Total
Flea bite
Yes
40 20 60
Flea bite
No
10 30 40

 

50 50 100

 

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