Isolating the direct effect of policy reforms: An illustration for the United States. To do this, one needs to derive a redistribution measure using the same population and distribution of market incomes before the reform (at time to) and after the reform (e.g., at time ti). Because populations change at the same time as policy, such measures are not directly observable. They can, however, be calculated using microsimulation models as these models can apply policy rules from different years to the same population.