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Model diagram elements : Variable
Variable
How to add a Variable symbol
See Adding node-type elements.
Interpretation
A variable is used to hold one or more values. The value or values come from a mathematical expression. The expression may simply be a number, or it may be a complex mathematical expression involving various variables, operators (such as + and -), functions (such as log or square root), and conditional elements. The value of a variable may vary during the course of a simulation, if it is calculated from other parts of the model that change over time, or it may be constant.
The term "variable" is used to refer to a specific type of model element. This single element can be used for a wide variety of purposes, each of which is referred to in a different way by some modellers. There is rich potential for confusion here, so the following table sets out the correspondence between how a Simile variable is used in a model, and how a modeller would interpret that use. (In case you are wondering why we don't have a number of model elements, one for each type of use: the answer is that this would lead to an unnecessary proliferation of element types. Also, you might wish to change the role of a variable as you build up a model, and you would not want to have to keep on deleting one symbol and replacing it by another.)
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Modelling use |
Set-up of "variable" |
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No influence arrows pointing to it. One or more influence arrows pointing from it. Value is a numeric constant or value is not supplied and "Fixed parameter" radio button is selected. |
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No influence arrows pointing to it. One or more influence arrows pointing from it. Value is a numeric constant (representing initial slider position). "Variable parameter" radio button is selected. |
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Exogenous variable: this is a variable whose value changes during a simulation run, and which influences the value of other variables, but which is not itself influenced by other variables. Typically used for climatic inputs, such as temperature or rainfall. |
No influence arrows pointing to it. One or more influence arrows pointing from it. Value is some function of simulation time (i.e. involves the built-in function time). |
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Intermediate variable, also referred to as a derived variable |
One or more influence arrows pointing to it. One or more influence arrows pointing from it Value is a function of the variables influencing it and also possibly of model properties such as current time |
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Output variable: typically, this is used to report on some aspect of model behaviour (e.g. the ratio of two compartments). |
No influence arrows pointing from it. Otherwise as intermediate variable |
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Attribute of an object: there is only sense in doing this if the variable is inside a multiple-instance submodel, with different instances having different values. E.g. the x-coordinate or the species type of each of many trees. |
No influence arrows pointing to it. No influence arrows pointing from it. |
Parameter (a coefficient in an equation): e.g. the reproductive rate per individual animal. Could also be a site constant: e.g. elevation above sea level. Its value will remain constant throughout a simulation run.
Input lever: a Rules
- A variable symbol may have zero or more influence arrows pointing to it.
- A variable cannot have influence arrows pointing to it if it is a "file parameter".
- A variable symbol may have zero or more influence arrows pointing from it.
- A variable may not have other kinds of arrow pointing to or from it.
In : Contents >> Graphical Modelling >> System Dynamics
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