How to: Contingency table analysis
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How to: Contingency table analysis |
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1. Create a contingency table From the Welcome or New table dialog, choose the contingency tab. If you are not ready to enter your own data, choose to use sample data, and choose any of the provided data sets.
2. Enter data Most contingency tables have two rows (two groups) and two columns (two possible outcomes), but Prism lets you enter tables with any number of rows and columns. You must enter data in the form of a contingency table. Prism cannot cross-tabulate raw data to create a contingency table. For calculation of P values, the order of rows and columns does not matter. But it does matter for calculations of relative risk, odds ratio, etc. Use the sample data to see how the data should be organized. Be sure to enter data as a contingency table. The categories defining the rows and columns must be mutually exclusive, with each subject (or experimental unit) contributing to one cell only. In each cell, enter the number of subjects actually observed. Don't enter averages, percentages or rates.
3. Analyze From the data table, click If your table has exactly two rows and two columns: Prism will offer you several choices:
We suggest you always choose Fisher's exact test with a two-tail P value. Your choice of additional calculations will depend on experimental design. Calculate an Odds ratio from retrospective case-control data, sensitivity (etc.) from a study of a diagnostic test, and relative risk and difference between proportions from prospective and experimental studies. If your table has more than two rows or two columns If your table has two columns and three or more rows, choose the chi-square test or the chi-square test for trend. This calculation tests whether there is a linear trend between row number and the fraction of subjects in the left column. It only makes sense when the rows are arranged in a natural order (such as by age, dose, or time), and are equally spaced. With contingency tables with more than two rows or columns, Prism always calculates the chi-square test. You have no choice. Extensions to Fisher's exact test have been developed for larger tables, but Prism doesn't offer them. 4. Review the results Interpreting results: relative risk and odds ratio Interpreting results: sensitivity and specificity |
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