Assuming that the rejection region is placed in the correct tail,the power of detecting an effect is larger for one-tailed tests compared with two-tailed tests.

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This is true. One-tailed tests have a smaller area of rejection compared with two-tailed tests, therefore, the probability of rejecting the null hypothesis is larger in one-tailed tests. This results in a higher power to detect an effect in one-tailed tests. However, one-tailed tests are only appropriate when the direction of the effect is known in advance.

Assuming That the Rejection Region Is Placed in the Correct