[問題] 關於二項轉常態的微調問題
各位板友大家好，請教各位一個統計問題
題目很簡單，大概是這樣：
30% of credit card purchases at a large
department store are for amount above $100.
A random sample of 100 credit card
purchases is selected. What proportion in the
sample between 20%~30% of the purchases over
$100 ?
這題轉成常態去算:
p=30%=0.3
P(100×20%≦X≦100×30%)=P(20≦X≦30)
=P((20-0.5-100×0.3)/(√(100×0.3×0.7))≦Z≦(30+0.5-100×0.3)/(√(100*0.3
*0.7)))
=P(-2.29≦Z≦0.11)=0.5328
然而書上的答案卻沒微調:
P((20-100×0.3)/(√(100×0.3×0.7))≦Z≦(30-100×0.3)/(√(100×0.3×0.7)))
=P(-2.18≦Z≦0)=0.4854
請問大家到底哪個是對的呢？
希望大家能幫忙解答一下，謝謝！感激不盡！！！
P.S:數學符號不太會打，亂亂的還請各位見諒！
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※ 編輯: Korb 來自: 125.224.33.110 (10/04 00:14)