If Q is a set of consecutive integers, what is the standard

From the F.S 1,consider the series [-10,-8,-9......0,1,2,3....9,10]. We have a fixed value for the S.D for the given series. Also, S.D. doesn't change if we add a constant across. Thus, we could add a constant 'a' to the above series --> The value of S.D will not change, irrespective the value of a.Sufficient.

From F.S 2, all we know is that the median is 20 and the series is of consecutive integers. For 19,20,21 the S.D would be different than that for 18,19,20,21,22.Insufficient.

A.

Bunuel just one question what will be the SD of a set of (-10, -9 ,......0....9,10) and what will be the SD of (1,2,......20,21)..Also ranges of both sets.


Sorry for asking stupid questions but it confused me here.

Thanks,
Abhinav

The two sets {-10, -9, ..., 10} and {1, 2, ..., 21} have the same standard deviation and the same range:

SD = \(\sqrt{\frac{110}{3}}\);

RANGE = (largest)-(smallest) = 10-(-10) = 20 or 21-1=20.

Thanks everyone who took initiative to explain.

I'm also posting an explanation from Manhattan GMAT and I hope none of us will get this or any other Standard Deviation Question wrong in future.

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The procedure for finding the standard deviation for a set is as follows:

1) Find the difference between each term in the set and the mean of the set.

2) Average the squared "differences."

3) Take the square root of that average.

Notice that the standard deviation hinges on step 1: finding the difference between each term in the set and the mean of the set. Once this is done, the remaining steps are just calculations based on these "differences."

Thus, we can rephrase the question as follows: "What is the difference between each term in the set and the mean of the set?"

(1) SUFFICIENT: From the question, we know that Q is a set of consecutive integers. Statement 1 tells us that there are 21 terms in the set. Since, in any consecutive set with an odd number of terms, the middle value is the mean of the set, we can represent the set as 10 terms on either side of the middle term x:

[x – 10, x – 9, x – 8, x – 7, x – 6, x – 5, x – 4, x – 3, x – 2, x – 1, x, x + 1, x + 2, x + 3, x + 4, x + 5, x + 6, x + 7, x + 8, x + 9, x + 10]

Notice that the difference between the mean (x) and the first term in the set (x – 10) is 10. The difference between the mean (x) and the second term in the set (x – 9) is 9. As you can see, we can actually find the difference between each term in the set and the mean of the set without knowing the specific value of each term in the set!

(The only reason we are able to do this is because we know that the set abides by a specified consecutive pattern and because we are told the number of terms in this set.) Since we are able to find the "differences," we can use these to calculate the standard deviation of the set. Although you do not need to do this, here is the actual calculation:

Sum of the squared differences:
10^2 + 9^2 + 8^2 + 7^2 + 6^2 + 5^2 + 4^2 + 3^2 + 2^2 + 1^2 + 0^2 + (-1)^2 + (-2)^2 + (-3)^2 + (-4)^2 + (-5)^2 + (-6)^2 + (-7)^2 + (-8)^2 + (-9)^2 + (-10)^2 = 770

Average of the sum of the squared differences:
770/21 = 36 2/3

The square root of this average is the standard deviation: ≈ 6.06

(2) NOT SUFFICIENT: Since the set is consecutive, we know that the median is equal to the mean. Thus, we know that the mean is 20. However, we do not know how big the set is so we cannot identify the difference between each term and the mean.

Therefore, the correct answer is A.

Sorry but this is way too complex to do in timed environment

Hi Abhinav,

One actually need not do all this calculation.
The reason I posted the explanation is because, understanding the concept of Standard Deviation will help us tackle all other questions on this topic.
GMAT tricks us by combining things such as - Standard Deviation and other concepts of statistics with Data Sufficiency.

In case you need further explanation, kindly let me know.

Target question: What is the standard deviation of Q?

Given: Q is a set of CONSECUTIVE integers

Statement 1: Set Q contains 21 terms.
NOTE: Standard Deviation measures dispersion (spread-apart-ness). As such, the actual values mean nothing compared to RELATIVE values.
For example, the set {1,2,3,4} has the SAME STANDARD DEVIATION as the set {6,7,8,9}

So, knowing that set Q consists of 21 CONSECUTIVE integers is SUFFICIENT.
The Standard Deviation of Q will be the same as the Standard Deviation of {1,2,3,4...20,21}

Statement 2: The median of set Q is 20.
There are several different sets that satisfy this condition.
For example, set Q could equal {19, 20, 21} or set Q could equal {18, 19, 20, 21, 22}
These two sets have DIFFERENT standard deviations.
So, statement 2 is NOT SUFFICIENT

Answer: A

You don't actually have to know what the median value of the set is because if you know the number of terms and the common difference- like in this sequence then you can know the s.d

x-2, x-1 ,x, x+1 .. etc

The SD is only restricted by the number of terms so we just need to know the number of terms

St 1

gives us this exactly

suff

St 2

clearly insuff

A

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