In the statistics context, what is covariance?

pls provide with sources if possible; thanks

Answer:
covariance is the measure of how much two variables vary together (as distinct from variance, which measures how much a single variable varies).

If two variables tend to vary together (that is, when one of them is above its expected value, then the other variable tends to be above its expected value too), then the covariance between the two variables will be positive.

On the other hand, if when one of them is above its expected value, the other variable tends to be below its expected value, then the covariance between the two variables will be negative.
Get your formula here:

http://mathworld.wolfram.com/covariance..
http://en.wikipedia.org/wiki/covariance.

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