Decoding the Data: Fisher鈥檚 Exact Test and McNemar鈥檚 Test
Christen Diel, DCLS, CC (NRCC), MLS (ASCP)CM, provides an overview of the Fisher's exact test and McNemar's test using a figure example.
Video Notes
Fisher’s exact test and McNemar’s test are commonly used to evaluate differences in proportions derived from 2×2 contingency tables in diagnostic microbiology studies. The choice between them is determined by whether the data are independent or paired.
Fisher’s exact test is appropriate for independent groups, as demonstrated in the example figure comparing the sensitivity and specificity of the Cobas Liat CT/NG/MG assay between symptomatic and asymptomatic patient populations. In this context, Fisher’s exact test assesses whether differences in detection rates between 2 separate populations are greater than would be expected by chance, particularly when error counts are small.
McNemar’s test is used for paired data, as shown in the example figure comparing 2 TB-IGRA assays performed on the same patient samples. This test focuses on discordant results and evaluates whether 1 method produces positive results more frequently than the other when applied to identical specimens.
Key Takeaways
- Independent patient groups require Fisher’s exact test, while paired samples require McNemar’s test.
- A statistically significant p-value indicates a difference in proportions, not diagnostic accuracy, agreement or clinical superiority.
Author Information
Christen Diel, DCLS, CC (NRCC), MLS (ASCP)CM, Medical Director & Clinical Consultant, Delta Pathology.
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