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

 It is currently 02 Sep 2015, 20:31

### 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

# If the positive number d is the standard deviation

Author Message
TAGS:
Senior Manager
Status: struggling with GMAT
Joined: 06 Dec 2012
Posts: 307
Concentration: Accounting
GMAT Date: 04-06-2013
GPA: 3.65
Followers: 11

Kudos [?]: 189 [0], given: 46

If the positive number d is the standard deviation [#permalink]  19 Feb 2013, 05:59
3
This post was
BOOKMARKED
00:00

Difficulty:

15% (low)

Question Stats:

77% (01:19) correct 23% (00:22) wrong based on 90 sessions
If the positive number d is the standard deviation of n, k and p then the standard deviation of n+1, k+1 and p+1 is

(A) d+3
(B) d+1
(C) 6d
(D) 3d
(E) d
[Reveal] Spoiler: OA

Last edited by Bunuel on 19 Feb 2013, 06:20, edited 1 time in total.
Edited the question.
Director
Status:
Joined: 24 Jul 2011
Posts: 709
GMAT 1: 780 Q51 V48
GRE 1: 1540 Q800 V740
Followers: 92

Kudos [?]: 382 [1] , given: 14

Re: If the positive number d is the standard deviation [#permalink]  19 Feb 2013, 07:42
1
KUDOS
If all the observations of a data set change by the same amount, the standard deviation remains the same.

The standard deviation of the new set is also therefore d.

_________________

GyanOne | http://www.GyanOne.com | +91 9899831738

Veritas Prep GMAT Instructor
Joined: 11 Dec 2012
Posts: 313
Followers: 91

Kudos [?]: 221 [2] , given: 66

Re: If the positive number d is the standard deviation [#permalink]  19 Feb 2013, 08:09
2
KUDOS
Expert's post
mun23 wrote:
If the positive number d is the standard deviation of n, k and p then the standard deviation of n+1, k+1 and p+1 is

(A) d+3
(B) d+1
(C) 6d
(D) 3d
(E) d

GyanOne is absolutely right about the answer, it is indeed E. I've often heard students say that standard deviation is their least favorite statistic. I, however, think that it should be the opposite. Calculating a square root may seem tedious to some, but the GMAT never asks you to do the actual calculations, so the questions always revolve around the concept. In this case, whether the numbers n, k and p are all increased by 1 or 100, or even 5,000,000, the answer remains the same. The only time the standard deviation is going to increase or decrease is if the numbers are multiplied (or divided, which is really just multiplication by 1/number) by some constant.

A standard deviation question is an opportunity to get easy points because the concept being tested will never be that hard. The important notions are that moving the numbers by a constant changes nothing, multiplying amplifies everything (as you would by pinching an iphone screen) and that sets with the same numbers have standard deviations of 0 (ex: 5,5,5,5).

Quick refresher to calculate standard deviation:
1) Take the average of all numbers.
2) Subtract the average from each element of the set
3) Square this difference (they're all positive now)
4) Add up all these numbers and divide by the number of elements
5) Take the square root of this number. (note: the square is the variance, not tested on the GMAT)

Again, you don't ever have to do this on the GMAT, but the concep can help unlock quick answers to standard deviation questions.
_________________
Verbal Forum Moderator
Joined: 10 Oct 2012
Posts: 629
Followers: 64

Kudos [?]: 773 [0], given: 135

Re: If the positive number d is the standard deviation [#permalink]  19 Feb 2013, 08:42
Expert's post
mun23 wrote:
If the positive number d is the standard deviation of n, k and p then the standard deviation of n+1, k+1 and p+1 is

(A) d+3
(B) d+1
(C) 6d
(D) 3d
(E) d

Maybe if the theory is not known, one of methods might be to plugin numbers. Assume n,k,p as 1,2,3. Initially, S.D =$$\sqrt{\frac{(1-2)^2+0^2+(3-2)^2}{3}}$$ = $$\sqrt{\frac{2}{3}}$$. Similarly, S.D for 2,3,4 will be $$\sqrt{\frac{(2-3)^2+0^2+(4-3)^2}{3}}$$ = $$\sqrt{\frac{2}{3}}$$

Thus its the same.

E.
_________________
Veritas Prep GMAT Instructor
Joined: 11 Dec 2012
Posts: 313
Followers: 91

Kudos [?]: 221 [0], given: 66

Re: If the positive number d is the standard deviation [#permalink]  22 Feb 2013, 08:00
Expert's post
vinaymimani wrote:
Maybe if the theory is not known, one of methods might be to plugin numbers. Assume n,k,p as 1,2,3. Initially, S.D =$$\sqrt{\frac{(1-2)^2+0^2+(3-2)^2}{3}}$$ = $$\sqrt{\frac{2}{3}}$$. Similarly, S.D for 2,3,4 will be $$\sqrt{\frac{(2-3)^2+0^2+(4-3)^2}{3}}$$ = $$\sqrt{\frac{2}{3}}$$

Thus its the same.

E.

I'm a big fan of plugging in a few numbers if the theory is a little vague. Instead of racking your brain trying to remember some obscure rule (like odd x odd = even/odd?), just try with numbers and you'll see the answer quickly (3 x 5=15, so the product of odd x odd must be odd).

Always a good suggestion to keep in mind that we can proceed this way, thanks Vinaymimani!
_________________
GMAT Club Legend
Joined: 09 Sep 2013
Posts: 6190
Followers: 345

Kudos [?]: 70 [0], given: 0

Re: If the positive number d is the standard deviation [#permalink]  18 Oct 2014, 23:15
Hello from the GMAT Club BumpBot!

Thanks to another GMAT Club member, I have just discovered this valuable topic, yet it had no discussion for over a year. I am now bumping it up - doing my job. I think you may find it valuable (esp those replies with Kudos).

Want to see all other topics I dig out? Follow me (click follow button on profile). You will receive a summary of all topics I bump in your profile area as well as via email.
_________________
Re: If the positive number d is the standard deviation   [#permalink] 18 Oct 2014, 23:15
Similar topics Replies Last post
Similar
Topics:
standard deviation 3 13 Jan 2011, 20:37
Standard deviation 1 03 Aug 2010, 19:05
1 The mean and the standard deviation of the 8 numbers shown 2 29 Jul 2010, 22:39
standard deviation 6 14 Feb 2010, 16:26
2 The mean and the standard deviation of the 8 numbers shown a 8 21 Apr 2007, 13:02
Display posts from previous: Sort by