A test statistic is a function of the sample data that follows a known distribution (such as t, z, or χ²) under the null hypothesis. Common examples include z = (x̄ - μ) / (σ/√n) for the z-test and t = (x̄ - μ) / (s/√n) for the t-test. The test statistic is compared to critical values or used to calculate a p-value.