Ventana software support forum

Hi everyone!

I'd like to model the relation between the motorization rate in an area and the car mindedness. I modelled the motorization rate as a variable (determined by vehicles and population, which are both stocks) and the car mindedness as a stock. I'd like to use an equation where the increase/inflow of car mindedness is determined by: (constant x increase motorization rate).

My question is whether it is possible to extract the increase of a variable. Because then I need to find the difference between two time steps. But how do I do this?

And if anyone has any better or alternative ideas on how to model this relationship I'd love to hear them.

Thank you all so much!

You can use DELAY FIXED to get the value of the stock at the previous time step.

You can use DELAY FIXED to compute a derivative, but I would recommend against that. Jay Forrester said "nature abhors a derivative" for a good reason: legitimate examples of direct perception of a rate of change are rare.

You could instead do something like the following:
motorization = cars/population
car mindedness = SMOOTH( motorization, car perception adjustment time )
Here the SMOOTH is shorthand for a stock adjusting toward a goal or indicated level; you could also make this explicit:
indicated car mindedness = f(motorization)
car mindedness adj rate = (car mindedness - indicated car mindedness)/car perception adjustment time
car mindedness = INTEG( car mindedness adj rate, initial car mindedness )

You should of course define units for all this.

Thank you both so much!