The first step with effect forecasting consists of an inventory of the direct effects of an alternative followed by the inventory of the indirect effects. For the latter, for each direct effect a list is made of its indirect effects and its relationships with them.
In first instance, these direct and indirect effects will be described qualitatively. It is however strongly advised to make them quantitative as early as possible:

• quantification of direct effects can be based upon experience with similar alternative or historic information. When these are not available, experiments (like scale models) and system models (e.g., economic models and ecological models) might be helpful;
• quantification of indirect effects is usually based on historic data. Experiments are most commonly not suitable, given its restricted scale and/or time horizon.

As said before, system models might be helpful for the quantification of direct effects, as it describes the character of a (part of a) system. The value of such a model for effect forecasting depends amongst other on the level of aggregation. When this level is high (many components of the system are clustered), then the value of the outcomes of the model is relatively low. In other words, the model to be used to describe a system should be at the same level as the project under study.