1. A method comprising: receiving an input distribution of one or more parameters of a pre-defined system and mutual correlations between the parameters; normalizing the parameter distributions and decomposing the mutual correlations to generate standardized independent parameter sets; generating a specific set of input values for sampling of the pre-defined system based on the standardized independent parameter sets; performing pre-defined system sampling on the specific set of input values to generate pre-defined system output values; performing orthogonal polynomial fitting on the specific set of input values, the pre-defined system output values, and the standardized independent parameter set to generate an SAP model; and using the SAP model to generate an output distribution of the pre-defined system.