# BMJ小小統計問題（5）：什麼是P value？

2021/09/05閱讀時間約 4 分鐘

British Medical Journal，英國醫學期刊（British Medical Journal，簡稱 BMJ），是一份同行評審性質的綜合醫學期刊，創始於1840年，是最古老的醫學期刊之一。2020年的IF=39.890，在醫學研究領域享有盛名。
BMJ自2008年9月開始至2015年由兩位流行病與統計學專家持續出了300多期Statistical question系列。在這個系列中，兩位學者每次出一道統計與流行病學有關的選擇題作答及解釋。本系列將精選Statistical Question，採用中英對照，有興趣的朋友們可進行回答。
BMJ統計問題(5)：下列針對P值的敘述何者正確？
Which one statement is true of the P value?
a) 它指虛無假設為真的機率 (It is the probability that the null hypothesis is true)
b) 它指對立假設為真的機率 (It is the probability that the alternative hypothesis is true)
c) 它指虛無假設錯誤的機率 (It is the probability that the null hypothesis is false)
d) 它指對立假設錯誤的機率 (It is the probability that the alternative hypothesis is false)
e) 它指當虛無假設為真，則比已知結果或更極端結果發生的機率 (It is the probability of the given result or more extreme results if the null hypothesis were true).
Statistical testing doesn’t quite do what clinicians would wish for. We are interested to know how near the truth a study result is likely to be. Standard statistics start from a the theoretical probability distribution and give answers to questions such as: “If this study were based on completely random data, how unusual would these actual results be?”

Answer a) is frequently assumed to be the practical interpretation of a P value. It is not strictly correct, but for most people (except under exam conditions) that will not matter much when working to this assumption.