Find all School-related info fast with the new School-Specific MBA Forum

 It is currently 27 Mar 2015, 00:12

### GMAT Club Daily Prep

#### Thank you for using the timer - this advanced tool can estimate your performance and suggest more practice questions. We have subscribed you to Daily Prep Questions via email.

Customized
for You

we will pick new questions that match your level based on your Timer History

Track

every week, we’ll send you an estimated GMAT score based on your performance

Practice
Pays

we will pick new questions that match your level based on your Timer History

# Events & Promotions

###### Events & Promotions in June
Open Detailed Calendar

# Mrs. Hopkins has two math classes, each with the same number

Author Message
TAGS:
CIO
Joined: 09 Mar 2003
Posts: 465
Followers: 2

Kudos [?]: 33 [0], given: 0

Mrs. Hopkins has two math classes, each with the same number [#permalink]  23 Jul 2004, 11:51
Mrs. Hopkins has two math classes, each with the same number of students. What was the standard deviation of all her math students' scores on an identical test she gave both classes?

1) The standard deviation of the test scores in her first math class was 2 points.

2) The standard deviation of the test scores in her second math class was 2 points.
Director
Joined: 05 May 2004
Posts: 577
Location: San Jose, CA
Followers: 2

Kudos [?]: 32 [0], given: 0

E
Means of the 2 samples can be different
Manager
Joined: 16 May 2004
Posts: 65
Location: columbus
Followers: 1

Kudos [?]: 4 [0], given: 0

guys,
I need help with my stats. Its been a long time sinse I studied SD, mean, mode etc .can u guys point me to material that I could go through (preferably on the internet) . appreciate all the help in this regard

regards
Senior Manager
Joined: 22 Jun 2004
Posts: 394
Location: Bangalore, India
Followers: 1

Kudos [?]: 2 [0], given: 0

Re: standard deviation [#permalink]  23 Jul 2004, 22:47
I think it is E.

The only inference from the stem is that the total mean of the two classes is the average of the two means. Unless we have some kind of relationship about the marks, it is tough to determine individual means from either (1) or (2) or combined and thus the total deviation.

SD = sqrt(sum of squares of the mean deviations/total number of samples)

What we need to find out is the mean of all the students of both the classes and then compute the SD. By the very nature of the above formula(sqrt) and the statistics(two same groups may have different means but their mean deviations sum might be the same), we cannot say that SD = SD1+SD2 or anything of that kind.

ian7777 wrote:
Mrs. Hopkins has two math classes, each with the same number of students. What was the standard deviation of all her math students' scores on an identical test she gave both classes?

1) The standard deviation of the test scores in her first math class was 2 points.

2) The standard deviation of the test scores in her second math class was 2 points.

_________________

Awaiting response,

Thnx & Rgds,
Chandra

CIO
Joined: 09 Mar 2003
Posts: 465
Followers: 2

Kudos [?]: 33 [0], given: 0

Yup, you're all right! It is E. And the various explanations are all right.

For this question, you need to know that standard deviation alone means nothing without the average it's describing. If the test scores' SD was 2 for both classes, we don't know if they each averaged 85, or if one class averaged 80 while the other averaged 90. Without information about the actual test grades of the various students, we can't know what the standard deviation is.

Bottom line, you can't do anything with SD except describe the average. You certainly can't combine different ones in any meaningful way.
Similar topics Replies Last post
Similar
Topics:
4 In Mr. Smith's class, what is the ratio of the number of boy 4 20 Aug 2012, 02:20
In a certain room each row of chairs has the same number of 4 12 Jul 2009, 11:51
A professor gave the same exam to two classes. The 200 4 19 Jun 2008, 04:03
Each correctional facility in a certain state has the same 6 22 Aug 2007, 03:38
A teacher gave the same test to two classes: A and B. The 3 09 May 2005, 04:50
Display posts from previous: Sort by