## Population Example with Conveyors

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aliakhavan89
Senior Member
Posts: 130
Joined: Sun Oct 21, 2018 7:09 am
Vensim version: DSS

### Population Example with Conveyors

I was reviewing this example:

https://vensim.com/documentation/mgu09_ ... population

And I noticed that if I set the fractional birth rate to zero, the stocks become negative though close to zero. I could not understand the root cause and a potential solution to this. I have also tried smaller dt and made all parameters to the power of 2 to minimize integration and rounding errors. Can you please help me understand what's happening?

Thank you very much!
Posts: 4658
Joined: Wed Mar 05, 2003 3:10 am

### Re: Population Example with Conveyors

There is no code in the model to stop the levels going negative.

I've attached a version that does stop them going negative. Look at the equations for "young deaths" and "young adult deaths".
Attachments
convey4.mdl
Advice to posters seeking help (it really helps us to help you)
http://www.ventanasystems.co.uk/forum/v ... f=2&t=4391

Units are important!
http://www.bbc.co.uk/news/magazine-27509559
LAUJJL
Senior Member
Posts: 1443
Joined: Fri May 23, 2003 10:09 am
Vensim version: DSS

### Re: Population Example with Conveyors

The use of a minimum in the 'young deaths' looks strange. Logically the maturation should calculate how many young people still alive are getting older; And this calculation should not be able to force the level to be negative. One must decide if the young people becoming young adults are still belonging to the young when they are maturing or are belonging already to the young adults people. This will change their death rate. To my opinion, the maturation equation may have to be changed to avoid the use of a minimum that may hide an error in the maturation equation. I am of course not sure of that but I have not the time to study that further on and propose a different formulation for the maturation equation.

JJ
aliakhavan89
Senior Member
Posts: 130
Joined: Sun Oct 21, 2018 7:09 am
Vensim version: DSS

### Re: Population Example with Conveyors

Thank you. But why does it happen? I thought that leakage was properly calculated and controlled within the conveyor function.

By the way, the outflows become negative in one calculation step.
Screenshot 2024-06-19 at 10.45.53 AM.png (149.78 KiB) Viewed 3758 times
LAUJJL
Senior Member
Posts: 1443
Joined: Fri May 23, 2003 10:09 am
Vensim version: DSS

### Re: Population Example with Conveyors

The two errors in the maturations happen just before the levels reach zéro. Some formulation errors generate strange behaviour in limit conditions. There is probably an error somewhere in the model.
There is too an error in the population view of the world 3 model where the error disappears if one sets the time step to 1 year.
If in the model convey4.mdl you set the time step to 1. both errors disappear.

JJ