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Locate the lost direction
The shortcut discards whether the test statistic is positive or negative. An upper alternative looks for positive evidence, while a lower alternative looks for negative evidence. The same absolute statistic can therefore imply a small or large one-sided probability.
Use a concrete counterexample
For z = −2, the two-sided normal p-value is about 0.0455. Half is about 0.02275, but the upper-sided p-value is about 0.97725. The shortcut would turn evidence pointing away from an increase into apparent evidence for it.
Reconstruct the intended tail
| Step | Check |
|---|---|
| Alternative | Was it upper, lower or two-sided before the result? |
| Statistic | What is its signed value? |
| Distribution | Is the null reference symmetric and continuous? |
| Tail | Which event is counted by the intended procedure? |
Avoid extending the shortcut to every procedure
Doubling a smaller tail is a particular symmetric convention. Do not apply it automatically to asymmetric distributions or discrete tests without reading the method definition. Preserve the originally planned analysis in the report.