Table 4.

Type I and Type II Errors

True Condition (In the Population)
Trial study results	Therapies are different	Therapies are not different
Therapies are different	True positive (correct decision)	False-positive (type I error) Probability = α (same as P value)
Therapies are not different	False-negative (type II error) Probability = β	True-negative (correct decision)

Normally, α is 5% or 0.05 (setting a P value) and power is (1 − β), often, 0.8 or 0.9 (80% or 90% power). P value and power are used in sample size calculations.