![]() ![]() I recommend you use Fisher's exact test when the total sample size is less than \(1000\), and use the chi-square or G–test for larger sample sizes. The most common use of Fisher's exact test is for \(2\times 2\) tables, so that's mostly what I'll describe here.įisher's exact test is more accurate than the chi-square test or G–test of independence when the expected numbers are small. (2013) actually had a third treatment, \(13\) people given vancomycin plus a bowel lavage, making the total data set a \(2\times 3\) table (or a \(3\times 2\) table it doesn't matter which variable you call the rows and which the columns). vancomycin data I'm using as an example is a \(2\times 2\) table. Fisher's exact test will tell you whether this difference between \(81\) and \(31\%\) is statistically significant.Ī data set like this is often called an "\(R\times C\) table," where \(R\) is the number of rows and \(C\) is the number of columns. The percentage of people who received one fecal transplant and were cured (\(13\) out of \(16\), or \(81\%\)) is higher than the percentage of people who received vancomycin and were cured (\(4\) out of \(13\), or \(31\%\)), which seems promising, but the sample sizes seem kind of small. The other nominal variable was outcome: each patient was either cured or not cured. One nominal variable was the treatment: some patients were given the antibiotic vancomycin, and some patients were given a fecal transplant. (2013) studied patients with Clostridium difficile infections, which cause persistent diarrhea. You want to know whether the proportions for one variable are different among values of the other variable. Use Fisher's exact test when you have two nominal variables. Our video tutorial uses a different data, and shows how to do Fisher’s Exact Test with a 2×2 contingency table.\) You may also be interested in our tutorials on: (1) exporting your SPSS output to another application such as Word, Excel, or PDF and (2) how to report Fisher’s exact test from SPSS in APA style. You should now know how to do Fisher’s exact test in SPSS, and also have an idea about how to interpret the result. But in a case like this one, where one of the variables has more than two possible values, it won’t run unless you choose the Exact option under the Exact Tests dialog. If your Crosstabs table is a 2×2 table – that is, if you’re comparing two categorical variables that have only two values each, then Fisher’s exact test will run automatically. Therefore, in this instance, we have no grounds to reject the null hypothesis that states there is no association between level of education and dog ownership. The Pearson Chi-Square test statistic (3.589) similarly does not reach significance. Normally this would not be considered significant (given an alpha level of. It’ll look something like this.Īs you can see above, the value of the Fisher’s exact test statistic is 3.286. ![]() You can view the test result in the SPSS output viewer. Press Continue, and then OK to run the test. Hit the Exact button (top right within the Crosstabs dialog), and choose the Exact option, leaving the test time limit as it is. You should now be back at the Crosstabs dialog. Press Continue when you’ve made the selection. To do this, click on Statistics, and choose the Chi-square option. The first stage in configuring SPSS to run Fisher’s exact test is to set up a chi square test. You can drag and drop, or use the arrows, as above. It doesn’t matter which variable goes into which box. One of these goes into the Row box, and the other into the Column box. If you have more than two, as in our example, you need to identify which of the two you want to test for independence. To start, click on Analyze -> Descriptive Statistics -> Crosstabs. The procedure for doing Fisher’s exact test in SPSS is similar to that used for the chi square test. Our data looks like this in the SPSS Data View. The result will appear in the SPSS output viewerįor this tutorial, we’re going to look at the association between level of education (High School, Graduate, PostGrad) and dog ownership (No Dog, Owns Dog).Press Continue, and then OK to run the test.Click on Exact, and then select the Exact option, leaving the test time limit as it is.Click on Statistics, select Chi-square, and then click on Continue. ![]()
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