13 Modelling
Modelling is an approach to impact evaluation which uses mathematical models to describe social and economic relationships and to infer causality from an intervention to an outcome, and/or between an outcome and its determinants. Models with more than one equation are most valuable, as they allow for both direct and indirect effects and also two-way relationships to be captured. Models can be used to examine the impact of a programme or policy by introducing them as an exogenous change in some of the variables, parameters or equations.