Different methods for scenario construction will obviously be necessary, depending on the type of scenario concerned.  Descriptive scenarios may be generated, for example, by generating a narrative based on the ideas of (and discussions with) the stakeholders.  Predictive or probabilistic scenarios usually require a significant amount of data gathering and analysis.  In each case, however, it is important to recognise that the scenarios, however constructed, represent a precursor to the stage of impact assessment; they define the assumptions and preconditions for the assessment that cannot, subsequently be changed (except by a full reiteration of the assessment).  For this reason it is essential that the conditions set down by the scenario are clearly and unambiguously specified and are meaningful (and plausible) within the context of the study.  If the assessment is to be informative, considerable care thus needs to be given to the process of scenario development, and scenario development can take up a considerable amount of time and effort.

To make the process of scenario construction easier, it is often useful to start with existing, ready-made scenarios.  This avoids duplication of effort, and it also helps to ensure greater consistency between different assessments. It is therefore advisable to begin by reviewing previous studies and seeing what scenarios they have used, and how they were developed (see links to examples under See also, below). 

In some areas, well-established scenarios have already been generated, often to address large, international issues, such as climate change, or as a framework for national planning and strategic development (e.g. national waste management scenarios).  Links to several European policy scenarios are given to the left.  In many cases, these existing scenarios provide a ready-made basis for assessment.  Nevertheless, they will often have to be adapted or further developed to match the specific issue being addressed; for example, by incorporating additional elements, by extending the timescale, or by rescaling the scenarios to a different geographic level.  What is most important is to retain, as far as possible, the broader context given by the existing scenarios so that the results of the assessment are consistent with what has been done before.

Various tools also exist to help users generate scenarios.  These range from relatively simple check-lists of things not to forget to rather sophisticated modelling and visualisation systems, that allow users to change key elements within the system, and estimate what the consequences might be.  Again, links to some of these scenario construction tools are given to the left.