Model Building Blocks
The Stock represents the main quantity that is to be accumulated (e.g., population, money in a bank account, water in a bathtub, etc.). The value increases or decreases over time. The stock value is what we want to track over time.
The Flow represents those actions or activities that cause the stock value to increase or decline over time. For population, the flows might be births and deaths; for money in a bank account, the flows could be depositing and withdrawing, etc. If the arrow points toward the stock, it causes the stock value to increase. If the arrow points away from the stock, it causes the stock value to decrease. A Bi-flow allows positive values to flow toward the white arrow and negative values to flow toward the black arrow.
A Bi-flow is always drawn with the white arrow pointing toward the stock icon. A bi-flow useful to use when a flow might change sign (positive to negative or vice versa) in the middle of a simulation run (as is often the case with problems that involve monitoring the height of an object thrown into the air).
A Converter is used to represent additional logic important to the model. It is often incorporated to define the logic that modifies the flow.
The Connector icon serves either as an information wire (dashed), or as an action wire (solid). This wire connects converters to the flow valve (or to other converters) so the flow equation has access to the parameter values (stored in a converter) or other modifying logic that allows the computer to calculate the flow value correctly.
Two Examples
The volume of water in a bathtub is increased by the constant inflow from the faucet and decreased by the draining process, defined here as an exponential process where a certain fraction of the current volume of water (stock value) is removed each minute.
A population is increased by births, an exponential process defined by multiplying the birth fraction by the current population. The population is decreased by deaths, also defined as an exponential process calculated by multiplying the death fraction by the current population.
