When "f" is a set of non-linear combination of the variables "x", an interval propagation could be performed in order to compute intervals which contain all consistent values for the variables. In a probabilistic approach, the function "f" must usually be linearized by approximation to a first-order Taylor series expansion, though in some cases, exact formulas can be derived that do not depend on the expansion as is the case for the exact variance of products. The Taylor expansion would be: