5.9. Uncertainty in a parameter value can, in principle, be described by a probability density distribution, either derived from observed statistics or more generally introduced as an expression of the degree of confidence in the accuracy of the value on the basis of expert judgement. Many uncertainties associated with a parameter that is not a numerical parameter can nevertheless be quantified and expressed as a probability distribution. An approach using probabilistic calculations derives the probability distribution for the outcome of the assessment from the probability density distributions for the parameters used in the assessment.