One-sided Test
In a one-sided (or one-tailed) test, the alternative hypothesis is directional: H₁: θ > θ₀ (right-tailed) or H₁: θ < θ₀ (left-tailed). The critical region and p-value are in one tail only. The critical value for a one-sided test at level α equals the critical value for a two-sided test at level 2α.
Real World
A manufacturer testing whether a new production process increases average battery life uses a one-sided test (H₁: μ > μ₀) because only an improvement — not a deterioration — is of commercial interest.
Exam Focus
Justify your choice of one-sided test in context before calculating — marks are awarded for the reasoning, not just the arithmetic.
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