If you have a data set and you want to compare the mean to a specific value (usually zero, but other values are possible), this is a one sample t-test. The hypothesis you are testing is H0: Mean = k
If the data set has two groups (such as male and female), and you want to compare the mean of group 1 to the mean of group two, this is a two sample t-test. The hypothesis you are testing is H0: Mean 1 = Mean 2
Both of the above are two-sided tests, because either the mean > k or mean < k would be a violation of H0
If you want to test to see if the mean is greater than or equal to some value k (as opposed to the mean being equal to k), this is a one sided test. For the two sample case, the one sided test would be H0: Mean1 > Mean 2
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Paige Miller