Learn in the textbookThe Framework of Hypothesis Testing
DefinitionA Type I error (第1種の誤り) is rejecting the null hypothesis when it is actually true.
ExplanationIt amounts to concluding “there is a difference” when in fact there is none. The probability of this error is kept from exceeding the significance level set before the test. Judging a new advertisement “effective” because a sample happened to show rising sales, when the ad actually did nothing, is one case of it.
AnalogyIt is like a healthy person being told at a checkup that they need further tests. Nothing is wrong, yet an abnormality is reported.
Memory hookIn Japanese it is nicknamed the “hasty person's error” (あわてものの誤り). Picture someone rushing to announce “there is a difference”.