Summary of the tiered approach

The amount of effort and detail devoted to analysing uncertainties should be proportionate to the needs of the problem, and so a tiered approach is recommended. 

Tier 0 exposure assessments, which are commonly used for first-tier screening purposes, do not require an analysis of uncertainty on every occasion, provided they include appropriate conservative assumptions or safety factors to take account of uncertainty. 

Higher-tier assessments do not require the quantification of every uncertainty. Therefore, a tiered approach is proposed where each individual source of uncertainty in an assessment may be treated at one of three tiers, beginning with qualitative approaches (Tier 1) and progressing to deterministic (Tier 2) or probabilistic approaches (Tier 3) when appropriate. Within a single uncertainty assessment, different sources of uncertainty may be treated at different tiers. Higher-tier methods are targeted on those uncertainties that have most influence on the assessment outcome. It is never practical to treat all uncertainties probabilistically therefore, even in a very refined assessment, some uncertainties will still be treated at the lower tiers. 

In the final assessment, some uncertainties may be quantified deterministically (Tier 2) or probabilistically (Tier 3). Sources of uncertainty that remain unquantified should be evaluated qualitatively (Tier 1). The overall assessment might be described as Tier 3 (because some sources of uncertainty are treated probabilistically), but it also contains elements at Tiers 1 and 2.  

In this chapter, both qualitative and quantitative approaches for characterizing uncertainty are described. We illustrate how these approaches apply and provide insight on the different tiers of an uncertainty analysis. 

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