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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