For the probability distribution functions (PDFs) of the input data, the user will supply the characteristics of the PDF (mean, variance, lower bound, upper bound, etc.). The expansion coefficients for the inputs are calculated using the characteristic information and the standard unit distribution for the particular PDF. The number of the terms in the expansion will depend on the type of the PDFs that characterize the model inputs. For a normal distribution, for example, the expansion will be defined with two coefficients, representing the mean and the variance (standard deviation), because these values can represent the input distribution in terms of the standard normal distribution.