首页 | 学习中心 | 注册 登录
返回列表 发帖
richer

Rank: 2

UID
217604 
帖子
179 
主题
139 
注册时间
2011-5-23 
最后登录
2012-9-12 
1# 跳转到 » 倒序看帖
打印
tT
发表于 2011-7-13 15:56 | 只看该作者

99% Confidence Interval

2011, standard, percent, normal
Are 99% of all observations 3 standard deviations away from the mean or 2.58 standard deviations away?

I read somewhere that 99 percent of observations under a normal distribution will be +/- 3 standard deviations from mean, but the formula for the 99% confidence inverval of a normal distribution is mean+/- 2.58*std dev


Im confused!



Edited 1 time(s). Last edit at Sunday, May 15, 2011 at 11:13AM by adq123.
收藏 分享

Jolyn

Rank: 2

UID
223297 
帖子
217 
主题
121 
注册时间
2011-7-11 
最后登录
2013-10-9 
2#
发表于 2011-7-13 15:56 | 只看该作者
2.58 is the exact number and usually one would say "more than 99% of observations are within 3 sigma of the mean"

Edit: Distributions which have tails as thin as normal are rare as rubies in the real world so 3 sigma is a pretty good real-world number.



Edited 1 time(s). Last edit at Sunday, May 15, 2011 at 12:01PM by JoeyDVivre.

TOP

ogoluwa

Rank: 2

UID
223249 
帖子
235 
主题
125 
注册时间
2011-7-11 
最后登录
2016-4-19 
3#
发表于 2011-7-13 15:56 | 只看该作者
I just came across a problem where they state that 99% of the observations are within the interval 200 and 400. Calculate the standard deviation, given a normal distribution.

Solution:

Mean:
(400+200)/2 = 300

Standard deviation = (400-300)/3


The solution doesn't take 2.58, rather it considers 3 standard deviations.

Which one do we choose on the exam: 2.58 or 3.

TOP

chetan86

Rank: 2

UID
223275 
帖子
251 
主题
133 
注册时间
2011-7-11 
最后登录
2013-8-23 
4#
发表于 2011-7-13 15:56 | 只看该作者
Yeh, I encountered exactly this question a month ago and had the same question. But you can answer that question using 2,58 sigma (as I remember you get closest answer from three choises). I hope there won't be such ambiguities on the exam.

TOP

JRossSter

Rank: 2

UID
223261 
帖子
237 
主题
103 
注册时间
2011-7-11 
最后登录
2013-12-5 
5#
发表于 2011-7-13 15:56 | 只看该作者
Is this a question on an official CFA mock exam, or on one of the third-party exams (Schweser, Stalla, etc.)?



Edited 1 time(s). Last edit at Sunday, May 15, 2011 at 02:31PM by Oyster.

TOP

dcfox83

Rank: 2

UID
223264 
帖子
251 
主题
127 
注册时间
2011-7-11 
最后登录
2016-4-19 
6#
发表于 2011-7-13 15:56 | 只看该作者
I think its Schweser I have seen this recently

TOP

MonkeyBusiness

Rank: 2

UID
223282 
帖子
241 
主题
126 
注册时间
2011-7-11 
最后登录
2013-8-21 
7#
发表于 2011-7-13 15:56 | 只看该作者
Doesn't this have to do with Hypothesis testing where for 2 tail hypothesis with 99%, the factor is 2.58...

Check page 605 for CFAI Vol 1.

TOP

Maddin

Rank: 2

UID
223256 
帖子
209 
主题
93 
注册时间
2011-7-11 
最后登录
2016-4-18 
8#
发表于 2011-7-13 15:56 | 只看该作者
Matori Wrote:
-------------------------------------------------------
> Doesn't this have to do with Hypothesis testing
> where for 2 tail hypothesis with 99%, the factor
> is 2.58...
>
> Check page 605 for CFAI Vol 1.


Same thing. Under the null hypothesis the test statistic is normal with mean 0.

TOP

kmf229

UID
223327 
帖子
324 
主题
105 
注册时间
2011-7-11 
最后登录
2013-10-21 
9#
发表于 2011-7-13 15:57 | 只看该作者
in all my reading, it was 2.58, so I think at the exam I'll play it safe by using 2.58

TOP

amqata

Rank: 2

UID
223313 
帖子
282 
主题
121 
注册时间
2011-7-11 
最后登录
2013-8-19 
10#
发表于 2011-7-13 15:57 | 只看该作者
Miss*Yiota Wrote:
-------------------------------------------------------
> in all my reading, it was 2.58, so I think at the
> exam I'll play it safe by using 2.58


Thats what I am doing...
I think some 3rd party notes/questions, might be rounding...

All I know is that 99% = 2.58 in my mind.

TOP

返回列表