Learn in the textbookThe Framework of Hypothesis Testing
DefinitionA Type II error (第2種の誤り) is letting slip a null hypothesis that is actually false, by failing to reject it.
ExplanationIt amounts to concluding “we cannot say there is a difference” when a difference really exists. Suppose a new study material does help, but the sample was too small for the difference to show, so its effect goes unnoticed. With other conditions equal, a larger sample makes this error less likely.
AnalogyIt is like a person whose illness has already begun being told at a checkup that nothing is wrong. A change that is really happening gets overlooked.
Memory hookIn Japanese it is nicknamed the “absent-minded person's error” (ぼんやりものの誤り). Picture someone daydreaming and missing a difference that is really there.