Ventana software support forum

Hi Guys,

If you have the time I'd appreciate some help with the outcomes of this model.

This is a model about population. Population extends in an are which is divided in three zones. POI is a variable which affects population growth and population movement between zones when POI values are very high (10).

Population in Zone 1 looks static and Population in Zone two doesn't change that much. Zone 3 is practically collapsed. It doesn't work with a carrying capacity of 8000 individuals as it should be so meanwhile I super exaggerated that number so to keep the model running.

Is there anything wrong that keeps population growth in that way? How can I make the carrying capacity in Zone 3 working with a value of 8000?

Many thanks!

I read Vensim users guide and I followed the rabbit model. But my model seems to be using slightly different equations and I still don't know what the problem is.



Regards,

Gaston

Hello Gaston,

The first thing I would do is fix the dimensional errors. You could be calculating the movement as people/month when you should be looking at people/year. If you put the dimensions in, this kind of error is easily caught.

There are other obvious errors as well. "movement z1 to z2" often takes the value 1.5, so you end up taking 30 people from "Total Z1 2000" when there are only 20 in the stock. The error is hidden as you have used "MAX (0, " in the equations.

So before going any further it would be a very good idea to do the following.
1. Make sure every model parameter has dimensions.
2. Make sure the model passes the dimensional check.
3. Remove all the "MAX ( 0 " as they are just hiding errors in this case.
4. Remove all numbers from equations and put them in separate constants (eg, "IF THEN ELSE(poi intensity=0, 0.5, IF THEN ELSE(poi intensity=5, 0.2, 0.01))" what are the figures 0.2 and 0.01, and what are they measured in).
5. Add some comments to equations to give an outsider an idea what you are trying to calculate.

Hope this helps.

Tony.