Given variables X and Y, the formula [read slowly] "quantity this value for X times Y" minus "quantity this value for X times this value for Y" gives the covariance of X and Y.
For a sequence of n random values, this quantity for values x-sub-1 through n can be calculated as [read slowly] this quantity for x-sub-1 times this quantity for x-sub-n by Wald's identity. Given variables X and Y, the formula [read slowly] "quantity this value for X times Y" minus "quantity this value for X times this value for Y" gives the covariance of X and Y. This quantity is defined as the (*) probability-weighted average of all possible values. The law of large numbers predicts that the mean will converge to - for 10 points - what anticipated long-run average result of an experiment?
For a sequence of n random values, this quantity for values x-sub-1 through n can be calculated as [read slowly] this quantity for x-sub-1 times this quantity for x-sub-n by Wald's identity.
expected value [or expectation value; prompt on average; prompt on mean or arithmetic mean]