# Variables

A variable is a pre-calculated or dynamically calculated value and is represented by an ellipse in the model.

Variables can be useful to synthesize data or supply calibration variables to your model. For instance, in a demographic model, a variable 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 variable 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 variable was connected with links to the necessary stocks; see the next section for more information on links):

**[Young Population]**+**[Middle Aged Population]**+**[Old Population]**

Variables can also be used to create forcing functions
which are based on the current time. Insight Maker 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 variable 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 variables. 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 variable.