Quote:

Originally Posted by **StJimmy666** Alright, debating with someone and I need a math person:
Given 2 sets of data (10 values each) so X and Y, how would you solve for the following:
(Summation Sign)XY
It's the sum of X times Y right? So how would you solve for it? |

If you are only referring to this "(Summation Sign)XY" then it should be the XY1+XY2+ etc etc. There is actually this

What does Σxy mean (sigma xy)? - Yahoo! Answers Quote:

Originally Posted by **super_gman** The way you have that equation written out, I would read it as SUM(X*Y). So you would multiply all the X's times their corresponding Y's, and add up all the products. Or, for the visually inclined:
X1Y1 + X2Y2 + X3Y3 + X4Y4 + X5Y5 + X6Y6 + X7Y7 + X8Y8 + X9Y9+ X10Y10
Of course, I say that under the assumption that the scores are somehow corresponding to one another, such as in a correlation. The original post wasn't 100% clear on that point (or perhaps I'm just not reading it properly). |

I agree. I don't really see why one would be calculating this because the two data sets have nothing to do with another.

Quote:

Originally Posted by **StJimmy666** AHA! I KNEW IT!
So my TA is indeed a dingbat and I'm not silly. This is great news.
The scenario is: The X set is the values of cigarettes smoked by 10 Americans daily, Y is the values of cigarettes smoked by 10 Europeans daily.
The questions were
sumXY (Sum of X times Y)
sumXsumY (Sum of X times Sum of Y)
sumXY - sumXsumY/N over N-1 (I hate math) |

Yeah. i don't see how the two data sets are related and would require this. Unless it is just to do it for maths sake. Might want to also check this out

Statistics
Another statistic is the covariance sigmaxy defined by:

sigmaxy = [(x1 - mux)(y1 - muy) + ... + (xn - mux)(yn - muy)]/n = (x1y1 + ... + xnyn)/n - muxmuy.

This is a measure of how much changes in x are associated with changes in y. If sigmaxy > 0 then x and y tend to increase or decrease together. If sigmaxy < 0, then y tends to decrease as x increases, and vice versa. If sigmaxy = 0, or is very small, then x and y tend to be independent of each other.

What class is this for? been awhile since I had to do any of this.

Might want to watch this

How To Calculate Covariance (Math)
I made a quick example in excel looking for the 3 explanations. I think this is accurate.

X Y XY

1 2 2

2 2 4

3 2 6

4 2 8

5 2 10

6 2 12

7 2 14

8 2 16

9 2 18

sumx 45

sumy 18

sumXY 90

sumXY/n 10

sumXY/n/n-1 1.25