A parameter is a pre-calculated or dynamically calculated
value and is represented by an ellipse in the diagram view.A parameter is a pre-calculated or dynamically calculated
value and is represented by an ellipse in the diagram view.
Parameters can be useful to synthesize data or supply calibration parameters to your model. For instance, in a demographic model, a parameter could be created to specify the birth rate. Alternatively, if you have a population model that has multiple age groups, you could dynamically calculate a parameter to add up all the age groups and determine the total population size using the following equation (you would have to make sure that the Total Population parameter was connected with links to the necessary stocks; see the next section for more information on links):
[Young Population]+[Middle Aged Population]+[Old Population]
Parameters can also be used to create forcing functions which are based on the current time. Simgua supports a number of functions that return the current time in the specified units. These functions are: seconds, minutes, hours, days, weeks, and years. The following equation could be used to create a parameter that returns the square of the current simulation time as measured in hours:
Hours^2
Sometimes it is hard to decide which parts of your model should be simulated using stocks and which parts with parameters. Generally stocks should only be used to model items that have inflows or outflows. If you have a stock in your model without either an inflow or an outflow, it would almost certainly be modeled better with a parameter.