• Fourfold (2×2) tables are used to describe enumeration data; they are concise and useful for summarizing many kinds of biomedical information. A familiar form is with two samples, each of which provides a two-valued outcome or observation, such as lived-died, normal-abnormal. These tables may be from research designs of several types: cross-sectional surveys, prospective models, retrospective models, and randomized clinical trials. Several methods of analysis of 2×2 tables are discussed, without the mathematical detail: χ2 test, critical ratio test, Fisher's exact test, and other less commonly used tests. The use of the relative risk ratio and the odds ratio to describe 2×2 tables is discussed. Some pitfalls in the use and interpretation of 2×2 tables are reviewed.