What is a 2 x 2 (read two by two) table, and what is it used for?
A 2 x 2 table is an easy way of summarizing and calculating all of the information about a diagnostic test. To make a 2 x 2 table for the Chlamydia example, draw a four-square table as follows, with test results + or - on the side, and true infection status + or - at the top. Then fill in the numbers from your data, putting true infections which the test labelled positive in the upper left corner, and so on.
| TRUTH | |||
| Disease Present | Disease Absent | ||
| TEST | Positive |
a |
b |
| Negative |
c |
d |
The 2 x 2 table allows easy calculation of the important values by the following formulas:
- sensitivity = a / a + c
- specificity = d / b + d
- positive predictive value = a / a + b
- negative predictive value = d / c + d
Tze-Wey Loong provided a very good explanation of sensitivity and specificity using visual aids, which may aid your understanding. See Tze-Wey Loong. Understanding sensitivity and specificity with the right side of the brain Br Med J 2003; 327: 716-719
