PresentationThe purpose of a risk assessment is to make a decision whether the risk of a given situation is acceptable, and, if not, how we can reduce it to a tolerable level. For many cases, this can be done in a semi-quantitative fashion. For more complex or problematic cases a quantitative approach is required. Anybody who has been involved in such a study is aware of the difficulties and pitfalls. Despite proven software many choices of parameters must be made and many uncertainties remain. The thoroughness of the study can make quite a difference in the result. Independently, analysts can arrive at results that differ orders of magnitude, especially if uncertainties are not included. Because for important decisions on capital projects there are always proponents and opponents, there is often a tense situation in which conflict is looming. The paper will first briefly review a standard procedure introduced for safety cases on products that must provide more or less a guarantee that the risk of use is below a certain value. Next will be the various approaches how to deal with uncertainties in a quantitative risk assessment and the follow-on decision process. Over the last few years several new developments have been made to achieve, to a certain extent, a hold on so-called deep uncertainty. Expert elicitation and its limitations is another aspect. The paper will be concluded with some practical recommendations
