Calculators5 min readUpdated: 2026-09-16
P-Value
Definition
A p-value is the probability of obtaining test results at least as extreme as the observed sample data, assuming the null hypothesis is true.
Detailed Technical Breakdown
Formulated by Sir Ronald Fisher, the p-value serves as the foundational decision metric in null hypothesis significance testing (NHST). A small p-value (typically p ≤ 0.05) indicates that the observed sample data would be highly improbable under the assumption of no effect, providing evidence to reject the null hypothesis in favor of the alternative hypothesis.
Key Technical Specifications
- Range: Bounded strictly between 0.000 and 1.000.
- Standard threshold: Alpha (α) = 0.05 (5% Type I error risk) or 0.01.
- Two-tailed tests evaluate both tails: P_two = 2 * min(P_left, P_right).
- Does NOT measure the probability that the null hypothesis is true.
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