  We consider models with topological sectors, and difficulties with their
Monte Carlo simulation. In particular we are concerned with the situation where
a simulation has an extremely long auto-correlation time with respect to the
topological charge. Then reliable numerical measurements are possible only
within single topological sectors. The challenge is to assemble such restricted
measurements to obtain an approximation for the full-fledged result, which
corresponds to the correct sampling over the entire set of configurations.
Under certain conditions this is possible, and it provides in addition an
estimate for the topological susceptibility chi_t. Moreover, the evaluation of
chi_t might be feasible even from data in just one topological sector, based on
the correlation of the topological charge density. Here we present numerical
test results for these techniques in the framework of non-linear sigma-models.
