2008 First Semester STATS课程官方问答贴

z-score

原帖由 mayigo 于 2008-4-27 22:28 发表
LZ不是很明白为什么不是直接用0.075 45.60,45.6不是分数吗?

我不知道你为什么选0.075, 中间的95%代表从2.5%到97.5%.

40.6(2.5%)到mean 60(50%)是19.4,

那么从50%到97.5%就是60 + 19.4 = 79.4

z-score

Leave it as two decimal places.

z-score

原帖由 天外飞猪 于 2008-4-27 18:16 发表



请问atm，你说的ip是不是=我们写的pr？还有108  Q4c还是不明白要怎么算/请问公式是什么呢？？

嗯，其实是一样的．我打IP是想和P(proportions)区别．

question 4

原帖由 z-score 于 2008-4-10 12:35 发表


(a) 我们不能算这个probability, 因为我们不知道passenger luggages的distribution.

(b) 4000 / 40 = 100.

(c) Yes you can. X bar ~ Normal(95, 11/sqrt(40))
IP(X bar > 100) use either R or SPS ...

367 楼的回复

z-score

原帖由 阴雨天 于 2008-4-27 19:15 发表
2007年 1st semester term test 第20题:
他让我算2-standard-error interval这是什么意思啊?是不是就是confidence interval? 可他有没给我how many percent confidence interval 怎么算啊? 谢谢

对的，是一个意思，two standard error interval基本是95%. 其实就是让你做一个+/- 2 * se(estimate)的interval.

z-score

原帖由 Nevne 于 2008-4-28 00:51 发表
anyone could help wiz Question 3 b) and c)

原帖由 t-multiplier 于 2008-4-18 15:29 发表


Questions 3

Do you think the explanations in 309 and 310 are helpful to you?

309:

ACT ~ Normal(21, 4.7)

SAT ~ Normal(500, 100)

Statement 1 - FALSE

21 - 2 * 4.7 = 11.6
...

420楼的回复．

z-score

原帖由 雨宁 于 2008-4-28 08:48 发表




HE IS RITE~

如果雨宁说Kurosagi对，Kurosagi一定对．

z-score

希望ATM回答了所有朋友的问题．不好意思，昨晚网络很慢，刷屏都不行．

akl

2006 test 的一道题，不太明白，阿童木能不能帮讲一讲，谢谢了。

Some research on polygraph tests (lie detector tests) suggests that an up-to-date
polygraph can detect about 70% of lies but will also incorrectly classify about
20% of true statements as lies. A local company uses a polygraph test in its
hiring process in the hope that it will only hire truthful applicants and reject
untruthful ones. The company estimates that about 90% of all its job applicants
are actually truthful.
Using these figures, of those rejected by the polygraph test, the percentage who
are wrongly rejected is:
(1) 18%
(2) 70%
(3) 20%
(4) 72%
(5) 7%

z-score

原帖由 akl 于 2008-4-28 11:01 发表
2006 test 的一道题，不太明白，阿童木能不能帮讲一讲，谢谢了。

Some research on polygraph tests (lie detector tests) suggests that an up-to-date
polygraph can detect about 70% of lies but will  ...

等等阿，今天上班有点忙．你先画出来表格，画出来表格就一目了然了．

kirstens

Let me try:
Sorry, I didn't notice it aint an assignment question. My answer to this question is 72%
Let
A=accepted by the test
R= rejected by the test
------------------------here P(A)+P(R)=1
L=lies in reality
T=true in reality
------------------------here P(L)+P(T)=1
according to the question:
P(R|L)=0.7
P(R|T)=0.2
P(T)=0.9
------------------------so you would know directly P(L)= 1-0.9 = 0.1
asking P(T|R)=?
HINT: rewrite the above equations as:
P(R|L)=P(RnL)/P(L) etc
and one more hint: P(R)=P(RnL)+P(RnT) as they form partition

so P(T|R)=P(TnR)/P(R)
from P(R|T)=0.2 >>P(RnT)/P(T)=0.2 >>P(RnT)=0.2P(T) where P(T)=0.9
from P(R|L)=0.7 >>P(RnL)/P(L)=0.7 >>P(RnL)=0.7P(L) where P(L)=0.1
and P(R)=P(RnL)+P(RnT)
so P(RnT)=0.18 and P(R)=0.25 >>P(T|R)=0.18/0.25=0.72

[ 本帖最后由 kirstens 于 2008-4-28 14:40 编辑 ]

z-score

原帖由 akl 于 2008-4-28 11:01 发表
2006 test 的一道题，不太明白，阿童木能不能帮讲一讲，谢谢了。

Some research on polygraph tests (lie detector tests) suggests that an up-to-date
polygraph can detect about 70% of lies but will  ...

table应该是这样的

　　　　　　　　Truths          Lies        Total
Polygraph truth             720                  30            750
   
Polygraph lies              180                  70            250

Total                         900             100           1000

180 / 250 = 72%

kirstens

haha, checked answer. we are both right

z-score

原帖由 kirstens 于 2008-4-28 14:42 发表
haha, checked answer. we are both right

~Kurosagi~

我感觉被无视了.....

kirstens

refer to #526, your request was answered.

阴雨天

原帖由 z-score 于 2008-4-28 08:49 发表


frequency table的standard deviation要用interval的中间数当作class value.

例如:
0 - 100     50

101 - 200  100

在计算器上输入data用50;50 M+然后150;200 M+

那比方说如果0-100的frequency是30呢? 在计算器上输入30遍50 M+?还是?

~Kurosagi~

終於交了.....

明天考試....

暈了我

z-score

原帖由 ~Kurosagi~ 于 2008-4-28 15:47 发表
我感觉被无视了.....

啊？为什么？

z-score

原帖由 阴雨天 于 2008-4-28 19:09 发表

那比方说如果0-100的frequency是30呢? 在计算器上输入30遍50 M+?还是?

没错，30遍50在计算器上是50 ; (shift ,) 30

z-score

原帖由 mayigo 于 2008-4-27 01:20 发表
A criminologist developed a test for predicting whether a teenager will become a
delinquent or not. Scores on the test can range from 0 to 100, with higher values
supposedly reflecting a greate ...

啊？magiyo, 我看错了。So sorry.

0.025 40.40

60 - 40.4 = 19.6

60 + 19.6 = 79.6

~Kurosagi~

謝謝 atm ^^

阴雨天

考完了~感觉还8错，心情舒畅！
谢谢阿童木。

spirithell

Chapter 9    Hypothesis Testing
STAT101 What dose it talk about?

z-score

It's talking about beliefs and how to reject it. Don't worry about it at this stage, if you can do the first 2 questions of the third assignment, you are on the right track.

z-score

你好，好的，明天起来之后我帮你看一看，希望不要介意。

z-score

原帖由 雨轩听风 于 2008-5-5 10:19 发表
阿童木哪去了?
明天DUE阿

啊？明天due阿？我今天毕业典礼，还没结束。Sorry.