Using human judgment in system dynamics models of social systems

This article addresses concerns about the validity of system dynamics models that rely on the quantification of human judgment. It reviews epistemological and semantic problems associated with scientific discourse, summarizes properties of four levels of scientific discourse (nominal, ordinal, interval, and ratio), and describes the judgmental conditions that must obtain when high-level (interval and ratio) mathematics are applied to human judgment. Two simple system dynamics models employing continuous and discrete limens are used to illustrate the dynamic consequences of inappropriate uses of human judgment in system dynamics models. The author concludes that unless we have specific evidence that the judgmental phenomena we are modeling behave according to additive, multiplicative, or averaging models, we can not assume we are dealing with anything other than ordinal phenomena.