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Re: PS questions about standard deviation. [#permalink]
09 Apr 2013, 01:52

1

This post received KUDOS

Expert's post

doe007 wrote:

Bunuel wrote:

I guess this is not real GMAT question as to answer this question with 100% certainty you should calculate SD for two sets and GMAT usually do not require actual calculation of SD. Though it's possible to eliminate 3 wrong answers at the beginning.

Mean is 4 and so are the means of all 5 pairs from answers choices.

A. (-1, 9) These two numbers are farthest from the mean and they will stretch the set making SD bigger

B. (4, 4) These two numbers are closest to the mean and the will shrink the set making SD smaller

C. (3, 5) Suitable option so far

D. (2, 6) Suitable option so far

E. (0, 8) These two numbers are also far from mean and they will also stretch the set making SD bigger.

So, when I looked at the options C and D I assumed that C is also too close to the mean and it will affect it more than D. So I ended with D and was correct. But still my logic eliminating C was not sure thing, without the calculations.

For the original set 0, 2, 4, 6, 8, standard deviation = 3.16227766

A) For the set -1, 0, 2, 4, 6, 8, 9, standard deviation = 3.872983346 B) For the set 0, 2, 4, 4, 4, 6, 8, standard deviation = 2.581988897 C) For the set 0, 2, 3, 4, 5, 6, 8, standard deviation = 2.645751311 D) For the set 0, 2, 2, 4, 6, 6, 8, standard deviation = 2.828427125 E) For the set 0, 0, 2, 4, 6, 8, 8, standard deviation = 3.464101615

[As the sample size is very small, SD's are calculated using formula for sample.]

Difference between original stdev and stdev of option D is 0.333850535 Difference between original stdev and stdev of option E is 0.301823955

Though option D has a very close call, closest to the original standard deviation is found in option E.

Correct answer is option E.

I believe this is not a real GMAT question and GMAC would not ask for such lengthy calculations.

Bunuel, you said correctly that answer to this question can be found through calculation only. I am always impressed by your explanations. For this question, however, something is amiss. I wish there is a simple way to find correct answer for this question.

That's not correct.

SD of {0, 2, 4, 6, 8} = ~2.83 (3.16 is sample standard deviation and 2.83 is population standard deviation, which is tested on the GMAT).

C. SD of {0, 2, 4, 6, 8, 3, 5} = ~2.45 --> difference=0.38 D. SD of {0, 2, 4, 6, 8, 2, 6} = ~2.62 --> difference=0.21 E. SD of {0, 2, 4, 6, 8, 0, 8} = ~3.21 --> difference=0.38

Re: PS questions about standard deviation. [#permalink]
09 Apr 2013, 02:22

Bunuel wrote:

That's not correct.

SD of {0, 2, 4, 6, 8} = ~2.83 (3.16 is sample standard deviation and 2.83 is population standard deviation, which is tested on the GMAT).

C. SD of {0, 2, 4, 6, 8, 3, 5} = ~2.45 --> difference=0.38 D. SD of {0, 2, 4, 6, 8, 2, 6} = ~2.62 --> difference=0.21 E. SD of {0, 2, 4, 6, 8, 0, 8} = ~3.21 --> difference=0.38

The correct answer is D.

Bunuel, it is true that Official Guide for GMAT Review mentions only the SD for population though Statistics encourages SD for sample for small set of numbers. In fact, during study, we were asked to use only SD for sample for calculations in quantitative finance. However, GMAT is the playground of GMAC and their rules will prevail!

I withdraw my stand here considering GMAT strategy.

Re: PS questions about standard deviation. [#permalink]
14 Oct 2013, 08:01

Bunuel wrote:

santy wrote:

Bunuel, First of all thanks for all the wonderful material that you compile and post here on this forum. I have been following lot of your math related posts for past few days. Your posts are great help in the gmat prep.

I was wondering if you have solutions for these PS SD questions? - specially to Q#8 & 9?

Q#9: E is a collection of four ODD integers and the greatest difference between any two integers in E is 4. The standard deviation of E must be one of how many numbers? (A) 3 (B) 4 (C) 5 (D) 6 (E) 7

Let the smallest odd integer be 1, thus the largest one will be 5. We can have following 6 types of sets:

CALCULATING STANDARD DEVIATION OF A SET {x1, x2, ... xn}: 1. Find the mean, m, of the values. 2. For each value x_i calculate its deviation (m-x_i) from the mean. 3. Calculate the squares of these deviations. 4. Find the mean of the squared deviations. This quantity is the variance. 5. Take the square root of the variance. The quantity is th SD.

Expressed by formula: standard \ deviation= \sqrt{variance} = \sqrt{\frac{\sum(m-x_i)^2}{N}}.

You can see that deviation from the mean for 2 pairs of the set is the same, which means that SD of 1 and 6 will be the same and SD of 2 and 5 also will be the same. So SD of such set can take only 4 values.

Answer: B.

Solutions and OA's for other questions are on previous pages.

Hope it's clear.

Is there a way to solve 9 using logic, as opposed to calculations? I got every other question right, based on the logic involved in SD, or knowledge of what it is, but for #9 I can't wrap my head around it. I chose A

Re: PS questions about standard deviation. [#permalink]
19 Oct 2013, 09:21

Bunuel wrote:

hamza wrote:

BUNUEL: Please share your logic for Q#1.

1. A set of data consists of the following 5 numbers: 0,2,4,6, and 8. Which two numbers, if added to create a set of 7 numbers, will result in a new standard deviation that is close to the standard deviation for the original 5 numbers? (A) -1 and 9 (B) 4 and 4 (C) 3 and 5 (D) 2 and 6 (E) 0 and 8

I guess this is not real GMAT question as to answer this question with 100% certainty you should calculate SD for two sets and GMAT usually do not require actual calculation of SD. Though it's possible to eliminate 3 wrong answers at the beginning.

Mean is 4 and so are the means of all 5 pairs from answers choices.

A. (-1, 9) These two numbers are farthest from the mean and they will stretch the set making SD bigger

B. (4, 4) These two numbers are closest to the mean and the will shrink the set making SD smaller

C. (3, 5) Suitable option so far

D. (2, 6) Suitable option so far

E. (0, 8) These two numbers are also far from mean and they will also stretch the set making SD bigger.

So, when I looked at the options C and D I assumed that C is also too close to the mean and it will affect it more than D. So I ended with D and was correct. But still my logic eliminating C was not sure thing, without the calculations.

What is the most efficient approach to choose between C and D here? They look pretty tentative both of them

Thanks Cheers!

J

gmatclubot

Re: PS questions about standard deviation.
[#permalink]
19 Oct 2013, 09:21