## COVARIANCE

It is a measure of joint variability of two variables X and Y.

Where X, Y are the observed values of the two variables and E[X], E[Y] are their arithmetic means.

The covariance is the average of the products and therefore is positive in the case of concordance, that is, if a positive or negative difference of X by E[X] correspond mainly difference of equal sign of Y by E[Y]. Conversely, it is negative in the case of a discrepancy between the signs of the differences between the two variables. The link between the two variables is defined direct in the first case, and reverse in the second case.

*Bibliografia*

Guseo R., Statistica, CEDAM, 2006.

Leti G., Statistica descrittiva, Il Mulino, 1999.

Editor: Giuliano DI TOMMASO - ASSONEBB