It considers the influence of input variability on output variability of a design solution. The question is "how sensitive does a small input change effect on the system output". For consistent input variability, the output variances have to be computed to determine the robustness of the design solution. The design solution is robust and insensitive to changing environment if the output variance is small. Otherwise, the design solution is not robust and sensitive if the output variance caused by the same input distribution is great. The robustness analysis let be reduced to the calculation and analysis of the variance of the product characteristic distibutions.