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Study Analysis: 2 x 2 Table or Chi-square Test

Lead Author(s): Jeff Martin, MD

Measuring Association

The simplest way to measure the association is to have a dichotomous outcome and dichotomous exposure variable as can be seen in the table below.

  • The proportions with and without disease are compared among those with and without the exposure.

2x2 Table

This is a conventional schematic of a what is called in epidemiology a 2 x 2 table. This table is used to illustrate basic measures of association.

Additional Exposures

  • Of course, exposures might have more than two levels and thus more than just yes/no categories, but the same principles would apply to the measures of association.

Table Layout

This 2 x 2 schematic is widely used, but exposure and disease can switch position as can the yes and no columns and rows.
  • It is important to notice how the table you are looking at has been arranged.


At the beginning of a cohort study or randomized trial, we know who is exposed or treated (a + b) and who is not exposed or not treated (c +d).

These are referred as the margins of the 2x2 table.

Topic revision: r5 - 05 Jun 2009 - 15:51:19 - MaryB?

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