A test statistic is a numerical value that is calculated from a sample of data and is used to test a hypothesis about a population parameter. It is typically used in hypothesis testing to determine whether an observed sample statistic is significantly different from what would be expected under a null hypothesis. The test statistic is compared to a critical value or p-value to determine whether to reject or fail to reject the null hypothesis. The choice of test statistic depends on the type of data being analyzed and the hypothesis being tested. Common test statistics include t-tests, chi-squared tests, and F-tests.