A researcher plans a one-tailed test instead of a two-tailed test, keeping alpha at 0.05. Compared to a two-tailed test, the one-tailed test will:

• A Have a higher risk of Type I error
• B Require a larger sample size for the same power
• C Split the rejection region equally into both tails
• D Be easier to reject the null hypothesis if the effect is in the predicted direction ✓

Explanation

A one-tailed test places the entire rejection region in one tail, making it easier to reject the null when the effect goes in the predicted direction. Type I error remains at alpha (0.05), not higher. Option C describes a two-tailed test. Option B is wrong because a one-tailed test requires a smaller sample for the same power, not larger.

Reference: Park's Textbook of Preventive and Social Medicine, 26th ed.

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