Which statement describes a one-tailed (directional) test?

Which statement describes a one-tailed (directional) test?

• A. Tests for a directional effect, increasing power by concentrating the critical region in one tail.
• B. Tests for an effect in either direction regardless of direction.
• C. A one-tailed test reduces the sample size required for a given power.
• D. A one-tailed test uses a two-tailed p-value.

Answer: (A)

A directional (one-tailed) test focuses on detecting an effect in a specific direction. By placing the entire rejection region in a single tail of the distribution, you allocate all of your alpha to that direction. This makes it easier to reject the null when the true effect goes in that direction, increasing the test’s power for detecting that specific outcome. For a fixed significance level, the boundary you must cross is less extreme in a one-tailed test than in a two-tailed test, so more of the distribution’s mass lies beyond the cutoff in the chosen direction. The trade-off is that if the effect occurs in the opposite direction, the test has little or no power to detect it. This description matches a one-tailed test that concentrates the critical region in one tail to boost power for a directional effect.

A directional (one-tailed) test focuses on detecting an effect in a specific direction. By placing the entire rejection region in a single tail of the distribution, you allocate all of your alpha to that direction. This makes it easier to reject the null when the true effect goes in that direction, increasing the test’s power for detecting that specific outcome. For a fixed significance level, the boundary you must cross is less extreme in a one-tailed test than in a two-tailed test, so more of the distribution’s mass lies beyond the cutoff in the chosen direction. The trade-off is that if the effect occurs in the opposite direction, the test has little or no power to detect it. This description matches a one-tailed test that concentrates the critical region in one tail to boost power for a directional effect.