![]() If your degrees of freedom aren’t listed in the F Table, use the larger critical value.If you are given standard deviations, they must be squared to get the variances.For two-tailed tests, divide alpha by 2 before finding the right critical value.Right-tailed tests are easier to calculate. The larger variance should always go in the numerator (the top number) to force the test into a right-tailed test. ![]() In addition, you’ll want to bear in mind a few important points: Plus, the samples must be independent events. fit the shape of a bell curve) in order to use the test. Your population must be approximately normally distributed (i.e. Several assumptions are made for the test. ![]() Therefore, your null hypothesis will always be that the variances are equal. In other words, you always assume that the variances are equal to 1. You always test that the population variances are equal when running an F Test. For example, if you had two data sets with a sample 1 (variance of 10) and a sample 2 (variance of 10), the ratio would be 10/10 = 1. If the variances are equal, the ratio of the variances will equal 1. The equation for comparing two variances with the f-test is: The result is always a positive number (because variances are always positive). You can find the F Statistic in the F-Table.īack to Top F Test to Compare Two VariancesĪ Statistical F Test uses an F Statistic to compare two variances, s 1 and s 2, by dividing them. The F statistic formula is:į Statistic = variance of the group means / mean of the within group variances.
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