  In a computer experiment the choice of suitable estimators to measure a
physical quantity plays an important role. We propose a new direct route to
determine estimators for observables which do not commute with the Hamiltonian.
Our new route makes use of the Hellmann and Feynman theorem and in a diffusion
Monte Carlo simulation it introduces a new bias to the measure due to the
choice of the auxiliary function. This bias is independent from the usual one
due to the choice of the trial wave function. We used our route to measure the
radial distribution function of a spin one half Fermion fluid.
