In the original work of Morris the two sensitivity measures proposed were respectively the mean, μ, and the standard deviation, σ, of Fi. However, choosing Morris has the drawback that, if the distribution, Fi, contains negative elements, which occurs when the model is non-monotonic, when computing the mean some effects may cancel each other out. Thus, the measure μ on its own is not reliable for ranking factors in order of importance. It is necessary to consider at the same time the values of μ and σ, as a factor with elementary effects of different signs (that cancel each other out) would have a low value of μ but a considerable value of σ that avoids underestimating the factors.