I am trying to calculate the variance: $$ \langle(\bar{O}-<O>)^2\rangle $$ of the Monte-Carlo estimator $$ \bar{O}=\frac{1}{M}\sum_{m=1}^M{O_m} $$ For uncorrelated samples.
In order to do so, I need to know the expression for the covariance between two samples (so I can use: $Cov(O_m,O_n)=0$) but looking at definitions of sample covariance online only confused me, since it all seems to assume two different pools of samples, and gives the "total" covariance between these two pools rather than the expression for covariances between two individual samples.