  We analyze the fluctuations in the case of mass reweighting for N_f=2 Wilson
fermions. We use a domain decomposition factorization of the fermion
determinant. Ratios of determinants are estimated stochastically. We study the
stochastic and the ensemble fluctuations as a function of the volume V and the
mass shift Delta m. With our result it is possible to estimate the cost and the
effectiveness of mass reweighting. In addition we introduce a stochastic
estimation for the one flavor case without using the square root.
