Before revision:  We compute the mean and variance of the number of tests per individual as a function of the pool sizes m=(m_1,\dots,m_k) in the first k stages;
After revision:  Is the infection probability p is not smaller than 1-3^{-1/3 compute the mean and variance of the number of tests per individual as a function of the pool sizes m=(m_1,\dots,m_k) in the first k stages;