It is vital that a well-defined conceptual model can
be realized by a macro-model (e.g., a Continuous System Simulation (CSS) model)
or a micro-model (e.g., an Agent-Based model or Discrete Event Simulation
model) and still produce mutually consistent results. The Full Potential CSS
concept provides the rules so that the results from macro-modelling become
fully consistent with those from micro-modelling. This paper focuses on the
simulation language StochSD (Stochastic System Dynamics),
which is an extension of classical Continuous System Simulation that implements
the Full Potential CSS concept. Thus, in addition to modelling and simulating
continuous flows between compartments represented by “real” numbers, it can
also handle transitions of discrete entities by integer numbers, enabling
combined models to be constructed in a straight-forward way. However,
transition events of discrete entities (e.g., arrivals, accidents, deaths)
usually happen irregularly over time, so stochasticity often plays a crucial
role in their modelling. Therefore, StochSD contains powerful random functions
to model uncertainties of different kinds, together with devices to collect
statistics during a simulation or from multiple replications of the same
stochastic model. Also, tools for sensitivity analysis, optimisation and
statistical analysis are included. In
particular, StochSD includes features for stochastic modelling, post-analysis of multiple simulations, and presentation of the results in statistical form.
In addition to making StochSD a Full Potential CSS language, a second purpose
is to provide an open-source package intended for small and middle-sized models
in education, self-studies and research. To make StochSD and its philosophy
easy to comprehend and use, it is based on the System Dynamics approach, where
a system is described in terms of stocks and flows. StochSD is available for
Windows, macOS and Linux. On the StochSD homepage, there is extensive material
for a course in Modelling and Simulation in form of PowerPoint lectures and
laboratory exercises.
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