Creating a 2×2 Contingency Table

Creating a 2×2 contingency table is very useful in calculating a variety of public health measurements, including sensitivity and specificity, negative and positive predictive value, risk ratios, attack rate ratios, and odds ratios.

A 2×2 table is actually a 3×3 table when you include the rows and columns for the totals. If you are setting up a table to measure the sensitivity and specificity of a test or its negative and positive predictive values, you should put the test results on the y-axis (rows) and the actual presence of disease on the x-axis (columns).

  Disease No Disease Total
Test (+) (a) (b) a + b
Test (-) (c) (d) c + d
Total a + c b + d a + b + c + d

The highlighted section is where you will enter the data for each corresponding cell. You can set up the table switching the rows and columns but you will generally see them set up in this format with test results on the y-axis and disease on the x-axis.

Setting up a table to measure the association of a risk factor or exposure is similar, with the outcome or disease on the x-axis and the presence of the risk factor or exposure on the y-axis.

  Disease No Disease Total
Exposure (+) (a) (b) a + b
Exposure (-) (c) (d) c + d
Total a + c b + d a + b + c + d

Note: You can set up the table differently but you will need to be cognizant of which numbers you are putting in your numerator and denominator for the measure you are calculating. For example:

  Exposure (+) Exposure (-) Total
No Disease (b) (d) b + d
Disease (a) (c) a + c
Total a + b b + c a + b + c + d


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