作者Korb (来时无久去无余)
看板Statistics
标题[问题] 关於二项转常态的微调问题
时间Sat Oct 4 00:08:12 2008
各位板友大家好，请教各位一个统计问题
题目很简单，大概是这样：
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 ?
这题转成常态去算:
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
然而书上的答案却没微调:
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
请问大家到底哪个是对的呢？
希望大家能帮忙解答一下，谢谢！感激不尽！！！
P.S:数学符号不太会打，乱乱的还请各位见谅！
--
※ 发信站: 批踢踢实业坊(ptt.cc)
◆ From: 125.224.33.110
※ 编辑: Korb 来自: 125.224.33.110 (10/04 00:14)