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Is the standard deviation of the set of measurements x1, x2,

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Is the standard deviation of the set of measurements x1, x2, [#permalink] New post 25 Jun 2012, 02:01
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The Official Guide for GMAT® Review, 13th Edition - Quantitative Questions Project

Is the standard deviation of the set of measurements x1, x2, x3, x4, ..., x20 less than 3 ?

(1) The variance for the set of measurements is 4.
(2) For each measurement, the difference between the mean and that measurement is 2.


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Re: Is the standard deviation of the set of measurements x1, x2, [#permalink] New post 25 Jun 2012, 02:02
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SOLUTION

Is the standard deviation of the set of measurements x1, x2, x3, x4, ..., x20 less than 3 ?

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}}.

(1) The variance for the set of measurements is 4. The variance is just the square of the standard deviation, so if variance=4 then SD=\sqrt{4}=2. Sufficient.

(2) For each measurement, the difference between the mean and that measurement is 2. So, we have that m-x_i=2, since N=20 then we know everything to calculate the standard deviation. Sufficient.

Answer: D.

For more on that subject check:
Math Book chapter on SD - math-standard-deviation-87905.html
PS questions on SD - ps-questions-about-standard-deviation-85897.html
DS questions on SD - lately-many-questions-were-asked-about-the-standard-85896.html
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PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: Is the standard deviation of the set of measurements x1, x2, [#permalink] New post 26 Jun 2012, 14:31
is SD < 3/

st. 1) SD = square root of variance (4) = 2
sufficient

st.2.) 2 <3
sufficient

Answer D
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Re: Is the standard deviation of the set of measurements x1, x2, [#permalink] New post 27 Jun 2012, 11:39
Its D..

1)square root of variance is standard deviation.. so it will be 2.. that is less than 3 ..sufficient

2) s.d is 2 .. that is less than 3.. sufficient..

so D is the answer..

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Re: Is the standard deviation of the set of measurements x1, x2, [#permalink] New post 29 Jun 2012, 03:28
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SOLUTION

Is the standard deviation of the set of measurements x1, x2, x3, x4, ..., x20 less than 3 ?

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}}.

(1) The variance for the set of measurements is 4. The variance is just the square of the standard deviation, so if variance=4 then SD=\sqrt{4}=2. Sufficient.

(2) For each measurement, the difference between the mean and that measurement is 2. So, we have that m-x_i=2, since N=20 then we know everything to calculate the standard deviation. Sufficient.

Answer: D.

For more on that subject check:
Math Book chapter on SD - math-standard-deviation-87905.html
PS questions on SD - ps-questions-about-standard-deviation-85897.html
DS questions on SD - lately-many-questions-were-asked-about-the-standard-85896.html
_________________

NEW TO MATH FORUM? PLEASE READ THIS: ALL YOU NEED FOR QUANT!!!

PLEASE READ AND FOLLOW: 11 Rules for Posting!!!

RESOURCES: [GMAT MATH BOOK]; 1. Triangles; 2. Polygons; 3. Coordinate Geometry; 4. Factorials; 5. Circles; 6. Number Theory; 7. Remainders; 8. Overlapping Sets; 9. PDF of Math Book; 10. Remainders; 11. GMAT Prep Software Analysis NEW!!!; 12. SEVEN SAMURAI OF 2012 (BEST DISCUSSIONS) NEW!!!; 12. Tricky questions from previous years. NEW!!!;

COLLECTION OF QUESTIONS:
PS: 1. Tough and Tricky questions; 2. Hard questions; 3. Hard questions part 2; 4. Standard deviation; 5. Tough Problem Solving Questions With Solutions; 6. Probability and Combinations Questions With Solutions; 7 Tough and tricky exponents and roots questions; 8 12 Easy Pieces (or not?); 9 Bakers' Dozen; 10 Algebra set. ,11 Mixed Questions, 12 Fresh Meat

DS: 1. DS tough questions; 2. DS tough questions part 2; 3. DS tough questions part 3; 4. DS Standard deviation; 5. Inequalities; 6. 700+ GMAT Data Sufficiency Questions With Explanations; 7 Tough and tricky exponents and roots questions; 8 The Discreet Charm of the DS ; 9 Devil's Dozen!!!; 10 Number Properties set., 11 New DS set.


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Re: Is the standard deviation of the set of measurements x1, x2, [#permalink] New post 29 Jun 2012, 03:55
Is the standard deviation of the set of measurements x1, x2, x3, x4, ..., x20 less than 3 ?

(1) The variance for the set of measurements is 4.
(2) For each measurement, the difference between the mean and that measurement is 2.

1 Statement:

s.deviation = sqrt(variance) = > thus, the std is 2. Sufficient;


2 Statement:

The standard deviation cannot be greater than the difference of observation and the mean of the population, (take min and max of the population compare to mean). Thus, we can safely assume that the std is less than 3. Sufficient.



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Re: Is the standard deviation of the set of measurements x1, x2, [#permalink] New post 19 Sep 2012, 01:15
Hi Bunuel,
I am of the opinion that calculation is not needed to prove D is sufficient.

Standard deviation of an observation is how far it is from the mean of the observations.
so by definition of SD, D is sufficient..

Please correct me if my understanding is wrong.
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Re: Is the standard deviation of the set of measurements x1, x2, [#permalink] New post 19 Sep 2012, 22:23
Bunuel wrote:
SOLUTION

Is the standard deviation of the set of measurements x1, x2, x3, x4, ..., x20 less than 3 ?

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}}.

(1) The variance for the set of measurements is 4. The variance is just the square of the standard deviation, so if variance=4 then SD=\sqrt{4}=2. Sufficient

(2) For each measurement, the difference between the mean and that measurement is 2. So, we have that m-x_i=2, since N=20 then we know everything to calculate the standard deviation. Sufficient.

Answer: D.

For more on that subject check:
Math Book chapter on SD - math-standard-deviation-87905.html
PS questions on SD - ps-questions-about-standard-deviation-85897.html
DS questions on SD - lately-many-questions-were-asked-about-the-standard-85896.html


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Re: Is the standard deviation of the set of measurements x1, x2, [#permalink] New post 20 Sep 2012, 01:06
I am of the opinion that calculation is not needed to prove D is sufficient.

Standard deviation of an observation is how far it is from the mean of the observations.
so by definition of SD, D is sufficient..

Please correct me if my understanding is wrong.
Regards,
Sach
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Re: Is the standard deviation of the set of measurements x1, x2, [#permalink] New post 24 Jan 2013, 04:33
For 2 to hold good,

say mean is 4 and for each of the 20 measurements to have a difference of 2 from the mean, all the measurements have to be either 2 or 6. .

Please correct me if my understanding is not correct.
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Re: Is the standard deviation of the set of measurements x1, x2, [#permalink] New post 19 Jun 2014, 16:37
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Re: Is the standard deviation of the set of measurements x1, x2,   [#permalink] 19 Jun 2014, 16:37
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