An experimental trial produces random variables X1 and X2 with correlation r = E[X1X2]. To estimate r, we perform n independent trials and form the estimate
Where X1(i) and X2(i) are samples of X1 and X2 on trial i. Show that if Var[X1X2] is finite, then 
1, 
,... is an unbiased, consistent sequence of estimates of r