# Comparison of two proportions

Command: | Tests Comparison of proportions |

## Description

Performs a Chi-squared test for the comparison of two proportions (from independent samples), expressed as a percentage.

MedCalc uses the **N−1 Chi-squared test** as recommended by Campbell (2007) and Richardson (2011).

This test is not performed on data in the spreadsheet, but on statistics you enter in a dialog box.

Note that when the total number of cases is less than 20, it may be more appropriate to perform Fisher's exact test.

## Required input

In the dialog box, you can enter the two proportions (expressed as percentages) and the total number of cases.

For example when the prevalence of a disease in one sample of a total of 120 patients is 80%, and the prevalence of the disease in another sample of a total of 210 patients is 70%, you will enter the following in the dialog box:

After you click Test, the results of the test are displayed.

## Results

- The difference between the two proportions and a 95% confidence interval (CI) for this difference; the CI is calculated according to the recommended method given by Altman et al. (2000).
- Chi-squared test and P value: when this P value is less than 0.05, the conclusion is that the two proportions indeed differ significantly.

In an optional *Comment* input field you can enter a comment or conclusion that will be included on the printed report.

## Literature

- Altman DG, Machin D, Bryant TN, Gardner MJ (Eds) (2000) Statistics with confidence, 2
^{nd}ed. BMJ Books. (p. 49) - Campbell I (2007) Chi-squared and Fisher-Irwin tests of two-by-two tables with small sample recommendations. Statistics in Medicine 26:3661-3675.
- Richardson JTE (2011) The analysis of 2 x 2 contingency tables - Yet again. Statistics in Medicine 30:890.