Advices |
I wish here to give you some advices that I think are important to properly build a model. |
The terms of the model |
We must write two terms.
The first terms should define : The second terms, sufficiently precise, should determine : An example of insufficiently precise terms which has tree solutions : 1) calculate the quantity of water contained in a tank filled by a tap that delivers (1 x t) liters. Temporality is as follows : 1 cycle of 5 seconds, with t = 1 second. First solution : Considering that the event type is discrete; we add the quantity of water filled per unit of time : Second solution : Considering that the event is continuous type; we calculate the surface area of the triangle whose base is the number of unit time being, and height is the corresponding number of liters : Third solution : Considering that the event is continuous type; we add the quantity of water filled per unit of time : - the quantity filled per unit of time is : (t -1)+(1x1/2) => (t -1)+0.5 liters Graphical representation : This terms is not precise enough because it does not specify the type of event for each flow : discrete or continuous. One might think that opening the tap is done gradually, but it could be done in stages, in fact, nothing stated in the terms. If you replace : One might think that the bank transfer is done in stages, i.e. all month-end, but it could be done more gradually, every day or every week, in fact, nothing stated in the terms. |
The temporality of the model |
It is important to initially define the temporality of the model and not to change it later. Indeed, many temporal parameters can be used as variables in the equations of flow. I remind you that in the software TRUE, the system time is a calendar time system and not a physical time system. |
The evolution of stocks between two time units |
In the software TRUE it is possible to several times update a stock between two units of time.
I do not think this feature exists in other systems dynamics software. In this example, the stock will change tree times between two units of time : Graphical representation : Same example with following change : - the flow F4 is performed at the same time as the flow F2 : Graphical representation : |