All assessments are comparative, in that they assess the differences between one situation (e.g. a policy option) and one or more others (e.g. conditions without the policy). These different situations - or the factors that lead to them - have to be prescribed before an assessment can be done. This is done by defining scenarios.

A scenario is a vision of the world as it might be. Scenarios are not intended to be predictions of what will happen, or accounts of what has happened, but plausible descriptions of what the world could look like under a specific set of assumptions. Scenarios thus play a vital role in any assessment, for they determine the nature of the comparisons which will be made, and thus the possible results of the assessment.

As such, the scenarios should reflect the interests and expectations of the stakeholders involved, and once analysis has been started, they should not be changed, for to do so would mean that the assessment is no longer faithful to these expectations. Scenarios thus provide the fixed basis on which the assessment is done.