Built-in functions : Statistics

Statistical functions produce variates from distributions, and generally produce different values at each point in the model where they are called.

There are up to three forms of each statistical function:

• The form with a _const suffix. This will produce a new value when the simulation is initialized or reset, or when a submodel instance containing it is created. This value stays the same until the end of the run, or until the submodel instance containing it ceases to exist.
• The form with a _var suffix (or no suffix). This will produce a new value on each time step for each instance where it occurs. The sequence of values for all such functions can be initialized using the Initialize pseudo-random tool. If there is no _const form of a function, the _const behaviour can be produced by wrapping this form in the at_init() function.
• The form with an extra argument. The last argument (an integer) serves as a seed for the values produced by that particular occurrence of the function. These will be the same each run, and independent of any other statistical functions in the model. For instance if it is used in a conditional submodel, it will start producing the same sequence of results each time an instance of the submodel comes into existence (assuming the seed value is the same).

If a statistical function only has one form, it behaves like the 'var' form.