Destroyer QR - std. dev. question

[dent2638]

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I'm having trouble figuring out how to calculate a standard deviation based on the solution given. Problem goes like this:

find mean and standard deviation of this data set (temps in Jan.):
5,10,15,8,13,17,9.

Mean=11, easy enough. But the std.dev. calculation has no explanation and looks like this:

(36+1+16+9+4+36+4)/7
15.142857
sqrt(15.142857)
3.89 or 3.9

ans: mean=11, std. dev=3.9

how did he get there, and is something like this fair game for the QR??

 

[JBarr29]

So first step is get the average.
Next you find the deviation from the average for each value given, and square the deviation.
So your average here is 11, and the first piece of data in your data set is 5. 11-5 = 6
6^2 = 36

11-10 = 1
1^1 = 1
....
take the summation of the squared values and divide by the number of data characters - 1.
Then take the square root of that number and that is the standard deviation.
i do think it is fair game. It is not that difficult once you know how to compute really, and wont take as much time as you think

So actually i think the explanation should look like this:
(36+1+16+9+4+36+4)/6
sqrt(17.6667)
4.2

Hope that helps

 

[azar87]

is that question from destroyer or math destroyer i remember doing sumn along those lines as well as a diff type of standard deviation question in those books. It is fair game because i actually had statistics questions on the QR and that seems to be the new trend in the DAT.

 

[Streetwolf]

Standard dev of a population is divided by n.
Standard dev of a (small to medium sized) sample is divided by n-1.

Unfortunately they are never usually clear about which they want. The answer choices will normally be spread out enough. Also I doubt this appears on a section that does not permit calculators.

 

[dent2638]

aha. thank you all - not too complex, I guess. It is from the regular destroyer - not math destroyer. Are there other statistics rules/formulas out there we should know - because the review material I have seen does not cover any.

so, statistics questions are out there on the current DAT? The new one rolls out when they start the calculator version in March.. I'm taking mine Feb 24 - mostly because I don't want to deal with a harder QR section.