Modelling of the feedback-loop structure of social, industrial, urban and other complex systems. Feedback loops are the closed paths that connect actions and their effects on the surrounding conditions, with the resulting conditions in turn coming back as information to influence further action. A systems dynamicist obtains information from groups of people who understand aspects of the system in question and assembles such information into a model that captures its essential structure.
Within the feedback loops of a system, the principles of system structure indicate that two kinds of variables will be found, levels and rates. The levels are the accumulations (integrations) within the system, whereas the rates are the flows that cause the levels to change.