The probability of obtaining an average value z is the product of the probabilities of obtaining arbitrary individual mean values xi, but with the constraint that the average is z. We can express this through the following expression
˜p(z)=∫dx1p(x1)∫dx2p(x2)…∫dxmp(xm)δ(z−x1+x2+⋯+xmm),where the δ-function enbodies the constraint that the mean is z. All measurements that lead to each individual xi are expected to be independent, which in turn means that we can express ˜p as the product of individual p(xi). The independence assumption is important in the derivation of the central limit theorem.