Different problems require different forms of assessment.  Not all merit a full, integrated impact assessment, either because they are simple enough to be done by other, more traditional means, or because the uncertainties involved would be too large (see IEHIA in relation to other forms of assessment).  Whether or not a full integrated assessment is merited may also not be clear when the issue is first identified.  During the Design stage, therefore, an important task is to carry out a screening of the issue, to determine whether the likely impacts are sufficient to warrant detailed assessment, and if so how to proceed.

Types of assessment

Integrated assessments may also vary greatly in their scope, structure and design, depending on the type of question that is being addressed and the needs of the users.  No formal classification of assessments has been developed and agreed, but a useful distinction can be made between three different approaches:

• diagnostic assessments - aimed at addressing the question of 'is there a problem' and if so 'from where does it derive, or where might we best intervene?'
• summative assessments - aimed at determining how well existing policies are performing;
• prognostic assessments - aimed at answering 'what if' questions, about future impacts (e.g. under different policy scenarios).

Assessment and scenarios

These distinctions are important, for each approach is likely to imply a somewhat different process of assessment.  In particular, different types of assessment imply the use of different types of scenario.

All assessments involve at least two scenarios – one describing some reference situation and one describing an alternative set of conditions.  In some cases, more scenarios may be defined, presenting further alternatives (e.g. different policy options).   The nature of the reference and alternative scenarios varies, depending on the type of assessment being done:

• In a diagnostic assessment, the reference scenario is usually a description of the world as it is now (the status quo), while the alternative scenario represents some counterfactual, ideal state – for example, how the world might look without the hazard of concern (e.g. with zero pollution) or if the hazards were at tolerable levels. 
• In a summative assessment, likewise, the reference scenario describes the current situation (i.e. with the policy or technology of interest), while the alternative scenario represents conditions without the policy or technology.
• In a prognostic assessment, the alternative scenarios describe the world as it might look in the future if certain changes are allowed to happen (e.g. new policy developments, technologies or environmental changes). The reference scenario may, again, describe the current situation, but (more strictly) should project this into the future, to show how the world would look if the proposed changes did not occur – i.e. a ‘business-as-usual’ scenario.