- Often an assessment will comprise a number of main parts (e.g. exposure and hazard in a chemical risk assessment) and smaller, subsidiary parts (e.g. individual parameters, studies, or lines of evidence within the exposure or hazard assessment).
- The uncertainty analysis may also be divided into parts. Assessors should choose at what level to conduct it:
- Evaluate all uncertainties collectively, for the assessment as a whole.
- Divide the uncertainty analysis into parts, which evaluate uncertainties separately in major parts of the scientific assessment (e.g. exposure and hazard in a risk assessment). Then, combine the parts of the uncertainty analysis and include also any other identified uncertainties that relate to other parts of the scientific assessment as a whole, so as to characterise the overall uncertainty.
- Divide the uncertainty analysis into still smaller parts, corresponding to still smaller parts of the scientific assessment (e.g. every input of a calculation or model). Evaluate uncertainty collectively within each of the smaller parts, combine them into the main parts, and combine those to characterise overall uncertainty for the whole assessment.
- If the uncertainty analysis will be divided into parts, assessors will need to combine them to characterise overall uncertainty. Assessors should define in advance how the parts will be combined, as this will increase transparency and rigour. It is recommended to use a conceptual model diagram (see glossary for explanation) to show how the parts will be combined. The parts may be combined by expert judgement (Section 12.6), or by calculation (Sections 13, 14 or 15) if assessors quantify the uncertainty for each part and can specify an appropriate quantitative or logical model to combine them. Calculation is likely to give more reliable results, but should be weighed against the additional work involved.
- Assessors should judge what is best suited to the needs of each assessment. For example, it may be more efficient to evaluate uncertainty for different parts separately if they require different expertise (e.g. toxicity and exposure). Evaluating all uncertainties collectively (first option in point (2) above) is generally quicker and superficially simpler but requires integrating them all subjectively by expert judgement, which may be less reliable than evaluating different parts of the uncertainty analysis separately, if they are then combined by calculation. For this reason, it is recommended to treat separately those parts of the assessment that are affected by larger uncertainties (identified by a simple initial prioritisation, see Section 8).
- When a part of the scientific assessment is treated separately in the uncertainty analysis, it is not necessary to evaluate immediately all of the uncertainties affecting it; some of them can be set to one side and considered later as part of the overall characterisation of uncertainty, if this is more convenient for the assessor. However, it is recommended that only the lesser uncertainties are deferred to the overall characterisation, since it will be more reliable to combine the larger uncertainties by calculation.
- When the scientific assessment includes a quantitative or logical model, assessors may find it convenient to quantify uncertainty separately for every parameter of the model. In such cases, it will still be necessary to identify additional uncertainties that are not quantified within the model, e.g. uncertainties relating to the structure of the model (see Section 7.2) and take them into account in the characterisation of overall uncertainty (Section 16). In other cases, assessors might find it sufficient to analyse all the uncertainties affecting a model collectively (simplest option in point (2) above), or for major parts of the model without separating the individual parameters (intermediate option in point (2)).