OG11th----PS(59)(70)

lufan

OG11th---PS(59)(70)
(59)The arithmetic mean and standard deviation of a certain normal distribution are 13.5 and 1.5 respectively.What value is exactly 2 standard deviation less than the mean?  (a)10.5  (b)11.0  (c) 11.5 (d)12.0  (e)12.5
(70)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 deviation from the mean?(a)12.0   (b)13.5  (c)17.0   (d)23.5   (e)26.5
这两道题到底求什么?看的一头雾水

yaoyao99

Both questions are about the definition of standard deviation.  Have you reviewed standard deviation, mean and median yet?

lufan

这我知道,就是标准差,平均数和中间数.但是没有明白题目要我求的是什么

colleen

I think it is a simple calculation, Understanding is an important key to solve this question.

The first question:13.5-2x1.5=10.5

The second one I'm not sure the five choices are right.

20+2.5X3=27.5

lufan

I understand the first one.

But the answer of the second one is A(12),I can't understand why

yaoyao99

more than 2.5 standard deviation from the mean:

1) 2 SD larger than mean: > 20.0 + (2.5 x 3)

2) 2 SD smaller than mean < 20.0 - (2.5 x 3)

lufan

“ 2 standard deviation less than the mean”

only means 2 SD smaller than mean ??

zhaoyak7

以下是引用lufan在2006-7-6 10:56:00的发言：

“ 2 standard deviation less than the mean”

only means 2 SD smaller than mean ??

6楼的思路已经很明确了，距离2、5倍方差以上，要么是下面的公式1，大于27.5，而大的已经超出答案范围了，小的12.5正好有一个12符合要求，就是答案

more than 2.5 standard deviation from the mean:

1) 2 SD larger than mean: > 20.0 + (2.5 x 3)

2) 2 SD smaller than mean < 20.0 - (2.5 x 3)

lufan

我明白了.感谢大家的帮助.3Q