@Andy : Can you be a bit more specific as to which problem or as a whole if you have problem on co-variance, then its my personal suggestion to watch some 10mins videos present on youtube. They explain it in a very easy manner and then you can pick it up from there...If any problem issues then I can help you out... As to the calculation sorry.. I am not quite familiar with the calc so trying still..
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Thanks & Regards
Abhishek Iyer
"If You Obey All the Rules, You Will Miss All the Fun. "