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MEI S1 Help needed!

Hi chaps!

June 2005 paper, I'm told to calculate the sample variance of some data. What I'm doing is finding the difference from the numbers from the mean (x - mean) then squaring that and adding them together, then dividing by the number of terms -1 so that leaves me with 645.875/19 which gives me 34. but the solution in the answers is this:

Variance = 1/19(22839 - 657^2/20) = 66.13

I don't have a clue where they're getting these numbers from!!!

Thanks in advance. :smile:

Here's the PDF: http://www.mei.org.uk/files/papers/s105ju_dighb6.pdf
Reply 1
Avatar for ttoby
ttoby
15
You're using the formula s2=∑(xi−μ)2n−1s^2=\dfrac{\sum (x_i - \mu)^2}{n-1} where mu is the mean. It might be that you made a mistake somewhere with the arithmetic, with all the squaring and adding.

There is also another formula s2=1n−1(∑xi2−(∑xi)2n)s^2=\dfrac{1}{n-1}\left( \sum x_i^2 - \dfrac{(\sum x_i)^2}{n} \right) which is probably the one they're using.
Reply 2
Avatar for monkeynut
monkeynut
OP
Thanks ttoby. I'm getting ridiculous answers with the formula I'm using, and I'm getting sick of doing massive amounts of tables and squaring. I'll try the second formula and see how that goes.

Cheers again :smile:
Original post by ttoby
You're using the formula where mu is the mean. It might be that you made a mistake somewhere with the arithmetic, with all the squaring and adding.

There is also another formula which is probably the one they're using.


Hey take a look at my question plz :frown:
Reply 4
Avatar for monkeynut
monkeynut
OP
It worked! :smile:

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