**Covariance:**

The covariance between two random variables X and Y is the expected value of the product of the variables´ deviations from their means. If there is a high probability that large values of X go with large values of Y and small values of X go with small values of Y, then the covariance between X and Y will be positive; if there is a high probability that small values of X go with large values of Y and large values of X go with small values of Y, then the covariance will be negative.