  We derive a formula to calculate the local change to the log of any density
of states for smooth real observables. Using this in Monte-Carlo simulations,
we are able to calculate the expectation value of the observable with a
precision often better than standard sampling. The method can be applied to
previously generated configurations, as long as the analysis uses the same
action used to generate the configurations. We show that for observables such
as Wilson line correlators, errors are reduced by up to 4 times.
