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Data & Analytics · Intermediate

One-Tailed Test

A hypothesis test in which the alternative specifies a direction (greater than or less than), so the rejection region lies in one tail.

Definition

One-tailed tests have more power for the stated direction but cannot detect effects in the other direction. Use them only when the direction is justified in advance.

Exam tip

If H1 says 'increase' or 'decrease', the test is one-tailed; if it says 'different', it is two-tailed.

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