Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:6:p10
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 6 (pt 10/12)
Character Range: 1076672–1079685

and explicit statement of related uncertainties (US EPA 1995c).
    * Characterising uncertainty in risk informs the stakeholders about the range of possible risks from an exposure. Risk estimates may sometimes diverge widely (NRC 2008).
    * Characterising the uncertainty in risk associated with a given decision informs the decision-maker about the range of potential risks that result from the decision (NRC 2008).
Uncertainty analysis is generally a qualitative process though in some cases it can be semi-quantitative or quantitative.

The first step should be a consideration of the CSM and what aspects of that model are uncertain and how that uncertainty has been accounted for.

The second most important part of the uncertainty assessment is an evaluation of the uncertainty and variability in the site characterisation data. Site characterisation data will always be limited by time, site constraints and budgets. However, the risk estimates based on even quite limited data can be fit for purpose if the exposure concentrations are a long way below (or above) toxicity reference values, which indicate that the risks are either very low or very high. Decision-making based on such uncertain but quite clear results is straightforward. Where risks are close to or slightly above unacceptable (the 'grey' zone), the issue of the uncertainty and variability in the site characterisation data becomes much more important and so the uncertainty assessment needs to be more detailed.

When assessing risks, uncertainty can arise from missing or incomplete information, be incorporated into the scientific theory affecting the ability of a model to make predictions, and result from uncertainty affecting a particular parameter, for example, sampling errors. Such uncertainty has the potential to cumulatively overestimate or underestimate risk during an assessment. An assessment of uncertainty is a part of the health risk assessment process and consequently needs to be addressed for each step of the risk assessment and for its cumulative effect from all of the steps.

There are three broad types of uncertainty (US EPA 1992):
    * Scenario uncertainty is uncertainty arising from missing or incomplete information such as descriptive errors, aggregation errors, errors in professional judgement, and incomplete analysis.
    * Parameter uncertainty is uncertainty affecting a particular parameter such as measurement errors, sampling errors, variability, and use of generic or surrogate data.
    * Model uncertainty is uncertainties in scientific theory affecting the ability of a model to make predictions.
NRC (2008) provides a detailed evaluation of the techniques currently provided for in US EPA guidance and concludes that although a number of usable methodologies are provided, it is unclear what level of detail is required to capture and communicate key uncertainties. A further comment is that quantitative methods suffer from the difficulty in sensibly quantifying all uncertainties, and