Using a 2*2 contingency table

Epidemiologists use a 2*2 contingency table to display the performance of a screening test compared to the gold standard test:

A 2*2 contingency table is a table having 2 rows and 2 columns. Generally, we focus on the four cells labelled ‘a’, ‘b’, ‘c’, and ‘d’.

In the above table, the test results are read from left to right across the row; and the disease status is read from top to bottom down the column. Those who actually have the disease are referred to as disease positive; those without disease as disease negative; those with a positive test result as test positive; those with a negative test result as test negative.

Therefore, the total number of disease positive people is obtained by simply looking at the total of cells ‘a’ and ‘c’, and is shown as ‘Total Disease +ve’. Similarly, the sum of cells ‘b’ and ‘d’ gives us the total number of disease negative people (that is the total number of people who actually don’t have the disease), and is shown as ‘Total Disease –ve’.

Likewise, the total number of test positive people is obtained by reading across the corresponding (Test results +ve) row (light grey), and is the sum of cells ‘a’ and ‘b’ (‘Total Test +ve’). Similarly, the total number of test negative people is obtained by reading across the next row (darker grey), and is the sum of cells ‘c’ and ‘d’ (‘Total Test –ve’).