首先睇下independent events既def.
Two events A and B are independent if and only if their joint probability equals the product of their probabilities: P(A∩B)=P(A)P(B)
跟住今日發現有個case計出黎照def睇係independent但覺得實際上又唔似係independent,
假設有4個波分別叫1,2,3,4號,會隨機抽1個number既波出黎
而家當event A係抽中1或2號,event B係抽中1或3號
咁既話P(A∩B)會係1/4,因為一定要抽中1號先得
P(A)係2/4=1/2,P(B)都係2/4=1/2,咁所以P(A)P(B)=1/4=P(A∩B)
咁好明顯表面上合乎左P(A∩B)=P(A)P(B)呢個要求
但其實諗深D會發現如果抽中1號波會同時令2件事一齊發生,抽中4號波又會令2件事一齊唔發生,
咁可唔可以話2件事其實係有關係(dependent)既呢?
又如果我而家total唔止4個波,變左有5個波分別係1-5號
咁既話P(A)就變左2/5,P(B)又係2/5,P(A∩B)會係1/5(因為要抽中1號)
今次就發現(2/5)(2/5)係唔等於1/5,所以5個波個case一定係dependent既
就算睇埋3個波,(2/3)(2/3)都唔等於1/3,亦都係dependent
咁點解唯獨是4個波個case要係independent?明明大家都係抽緊波對number姐
加上去睇埋wiki個def都話係if and only if,換句話說P(A∩B)=P(A)P(B)既話,
就一定要話佢係independent
但明明抽一個number出黎係會一次過影響哂event A同B既result,
咁唔係應該當成dependent合理d?加上試過3同5個波既case都真係dependent,
就算6,7,8...個波既case再計都一定係dependent,
咁點解4個波果陣因為咁岩計到P(A∩B)=P(A)P(B)就唔係?
覺得係/唔係既話都可以幫手解釋下你係點諗/點解?thanks