Formulating scenarios
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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.
The role of scenarios
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.
Scenarios and the causal chain
Assessments of systemic health issues can conveniently be structured on the basis of a causal chain, representing the key links between primary sources and ultimate impacts (see link to The causal chain for more information).
The position of scenarios in relation to the causal chain may vary. Some are exogenous (upper figure), and describe only the external context or driving forces that influence this chain (e.g. policy developments, technological changes). Most, however, are endogenous (lower figure) in that they describe links within the chain – for example, levels of (or changes in) source activities, releases, ambient concentrations and/or exposure.
As these figures show, the position of the scenario in relation to the causal chain fundamentally affects the character and scope of the assessment. This is because the scenarios give the fixed assumptions for the assessment. The assessment itself takes the conditions thus specified as given, and considers only the ‘downstream’ effects, as they work through the causal chain.
- Exogenous scenarios define the external conditions (i.e. context) but tell nothing direct about the changes that might occur within the causal chain. These all have to be deduced as part of the assessment – for example, by modelling the way the external influencers work their way from source to impact.
- Endogenous scenarios, in contrast, define some aspects of the causal chain – those which are specified as part of the scenario. The assessment is only concerned with estimating the subsequent implications . In the case of an emissions scenario, therefore, expected changes in emission are predefined, and the assessment focuses on estimating how these work through to affect environmental concentrations and exposures, and thence health effects. In the case of an exposure scenario (i.e. where changes in exposure are specified), the assessment only has to consider the link from exposure to health impact.
More detailed and encompassing scenarios might seem useful, since they define more clearly the conditions under which the assessment should be done. They do so, however, at the cost of predefining more of the results, and limiting the independence of the assessment. A balance needs to be sought, such that the scenario defines only what can legitimately be taken as given, while the assessment should explore and discover what is not known.
Types of scenario
Scenarios may be described and represented in different ways (see Table below). Which is used depends on the type of issue being considered (e.g. how complex it is), the needs and skills of the stakeholders involved in scenario development, and the type of assessment (i.e. diagnostic, summative or prognostic). Follow the links below to find more information on each type.
| Charasteric of approach | Prescriptive | Descriptive | Predictive | Probabilistic |
| Defined in terms of: | Outcomes | Contexts | Trends | Processes |
| Approach to system specification | Normative | Qualitative | Quantitative | Quantitative |
| Method of formulation | Expert judgement | Stakeholder consultation | Deterministic models or extrapolation | Stochastic models |
| Form of information | Assumptions (fixed states) | Narratives, pictures | Predictions of future states | Predictions of system behavioun |
