prep方差类型的题目，出现了好多次，遇到就晕。请高人指点

carolcai · 发表于 2008-7-5 15:59:00

If a certain sample of data has a mean of 20.0 and a standard deviation of 3.0, which of the following values is more than

2.5 standard deviations from the mean?

(A) 12.0

(B) 13.5

(C) 17.0

(D) 23.5

(E) 26.5

110.11364-!-item-!-187;#058&007646
The residents of Town X participated in a survey to determine the number of hours per week each resident spent watching television. The distribution of the results of the survey had a mean of 21 hours and a standard deviation of 6 hours. The number of hours that Pat, a resident of Town X, watched television last week was between 1 and 2 standard deviations below the mean. Which of the following could be the number of hours that Pat watched television last week?
(A) 30
(B) 20
(C) 18
(D) 12
(E) 6

118.
12240-!-item-!-187;#058&008164

For a certain examination, a score of 58 was 2 standard deviations below the mean, and a score of 98 was 3 standard deviations above the mean. What was the mean score for the examination?

(A) 74

(B) 76

(C) 78

(D) 80

(E) 82

[此贴子已经被作者于2008-7-5 16:02:30编辑过]

shiverson · 发表于 2008-7-5 16:21:00

记住与平均数mean的关系即可：

1。 20－2.5*3<所求<20+2.5*3

2。 21-2*6<所求<21-1*6

3。 58+2*所求＝98-3*所求

只要注意和mean的数学关系

irence_928 · 发表于 2008-7-5 17:23:00

请问一下LS,我不太明白你写的.可不可以解释一下which of the following values is more than

2.5 standard deviations from the mean?是什么意思.

为什么所求是在某个范围之间的??

carolcai · 发表于 2008-7-5 17:31:00

同问

zangyang81 · 发表于 2008-7-5 18:00:00

2.5 standard deviations from the mean

=离平均2.5个标准差,因为标准差为3，so 2.5个标准差为3*2.5。可以理解为在数轴上离20左右各7.5

tim_zyb · 发表于 2008-7-5 18:11:00

不太明白第一个……能否给解释解释？感觉有四个答案符合这个范围啊……

irence_928 · 发表于 2008-7-5 20:32:00

"2.5 standard deviations from the mean

=离平均2.5个标准差,因为标准差为3，so 2.5个标准差为3*2.5。可以理解为在数轴上离20左右各7.5"

问题是人家题目说more than啊!为什么不是理解为２０＋２．５＊３呢？

有没有哪位高手来指点一下啊？？

wendyx · 发表于 2008-7-5 21:39:00

其实就是问哪个数是在2.5倍标准差之外的。只有A

李傻傻 · 发表于 2008-7-5 21:48:00

那就是之前的这个公式错了:1。 20－2.5*3<所求<20+2.5*3

应该是:所求<20-2.5*3OR所求>20+3.5*3

之后的那两个公式应该是对的,但是楼上几位的意思,我是看懂了，呵呵

[此贴子已经被作者于2008-7-5 21:50:33编辑过]

tim_zyb · 发表于 2008-7-5 21:52:00

以下是引用wendyx在2008-7-5 21:39:00的发言：

其实就是问哪个数是在2.5倍标准差之外的。只有A

明白了！

prep方差类型的题目，出现了好多次，遇到就晕。请高人指点

prep方差类型的题目，出现了好多次，遇到就晕。请高人指点

所属分类: GMAT考试

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