An F-test is used to test if the variances of two populations are equal. It can be a two-tailed test or a one-tailed test. The two-tailed version tests that the variances are not equal. The one-tailed version only tests in one direction, i.e., whether the variance from the first population is greater than or less than (never both) the second population variance. The selection is determined by the problem or the hypothesis. For example, while testing a new process, one may only be interested in knowing if it is less variable than the old process.