We examine non-Boltzmann Monte Carlo algorithms used to study slowly relaxi
ng systems. By adding a simple bookkeeping step to the Metropolis algorithm
, we obtain statistical estimators of canonical macrostate probabilities. T
hese estimators enable a natural accumulation of statistics from simulation
s having different importance weights, enable temperature extrapolation wit
hout using energy to define macrostate labels, improve parallelization, and
reduce variance. We illustrate with an Ising model example.