When you test for differences between the means of two independent populations,you can only use a two-tail test.

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When testing for differences between the means of two independent populations, you have the option to use either a one-tailed or two-tailed test. The choice of test depends on your research question and hypothesis. A one-tailed test is appropriate if you are interested in whether the mean of one population is greater than or less than the mean of the other population in a specific direction. A two-tailed test is appropriate if you are interested in whether there is any difference between the means of the two populations, regardless of direction.

When You Test for Differences Between the Means of Two