increase of variable - car mindedness equation

[sya_ouniversity]

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!

[Administrator]

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

[tomfid]

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.

[sya_ouniversity]

Thank you both so much!