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System Dynamics Pitfalls and Pointers


Two essential model-building activities are verification and validation. Unfortunately these two terms are often used interchangeably, which leads to confusion. Verification and validation involve two distinctly different types of activities but which are frequently inseparable when it comes to system dynamics modelling. Our focus here is on verification. Simply put, verification helps us answer the question: "Are we building the model right"?
     In system dynamics modelling, verification is all about ensuring that the governing business rules have been correctly coded, that the structure in which those rules operate results in correct and sufficient replication of the reference modes of behaviour identified in an earlier stage (and specified as requirements for the model we are building).
     There are a variety of tests which might be conducted in the process of verifying a module or model. These include logical tests, extreme-value tests, and mass-balance tests. Mass-balance tests are designed to assure that physical flows do not violate the basic requirement for physical flows into a module, sector or model either accumulate or flow out. Mass-balance must be assured during every time-step of every simulation run.
     The method used to test the mass-balance of a Powersim Studio model is explained and demonstrated in . The corresponding Powersim Studio 2005 model is .


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