Greetings,
Let's say I have a mean value X with SD 1.5 (i.e. X ± 1.5) and another mean Y with SD 1.3 (Y ± 1.3). If I have to subtract those mean values, how can I do it for Standard deviation? (X-Y ± ??)
I would like to request experts to share ideas to solve this problem. Thank you.
Jochen Wilhelm
The SD of the difference of two random variables, X and Y, depends on the correlation their correlation. The variance of the difference is given by:
V(X-Y) = V(X) + V(Y) - 2 COV(X,Y)
If X and Y are idenpendent (don't correlate), then COV(X,Y) = 0 and the variance of the difference is just the sum of the variances.
And the SD is the square root of the variance, SD(X-Y) = sqrt(V(X-Y)).
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