Difference between one-sided and two-sided p-values
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If the alternative hypothesis is (say for test on one mean) Ha: µ < 50 or Ha: µ > 50 we have one-sided test. For Ha: µ > 50 the direction is to the right (right-tailed test). For Ha: µ < 50 the direction is pointing to the left (left-tailed test). On the other hand, the alternative hypothesis Ha: µ ≠ 50 does not specify the direction, towards right or left hence we have a two-sided test.
Additionally, for 5% alpha, here are the graphs related to rejection region of one-sided and two-sided tests.
Hope this helps.
The 365 Team