Ventana software support forum

Dear users,
I am modelling an economic growth model in vensim ple on windows 8. I have a variable called population that has been classified into childhood, young age, middle age and old age using an aging chain.Corresponding to population, I have another variable called total aggregate wealth that has been modelled as a coflow. So, i have young wealth (total savings of young people), middle wealth (total savings of middle aged people) and old wealth. Young and middle aged people have same propensity to save and old people dissave i.e. they consume off their wealth. Thus,Old wealth is total savings of people that they have done during their young and middle age.I am new to system dynamics and I am not very sure that i am formulating my total wealth correctly.Can someone please check whether my formulation?

One specific point that I think that I am doing wrong is that variable old age is not included anywhere when i am modeling total wealth as coflow.But i am not very clear how it can be included??

Hi Swati

Your coflow has some problems.

The young age and middle age savings look correct.

But the transfers are not.

Suppose that there is a strong epidemy during a semester and half of the middle age people die
, the people ageing after the semester will drop sharply and the transfers from middle to old too, which does not happen in your model.

To correct this one must make the transfers equal to the number of people ageing from one cohort to the next one multiplied by the total savings made so far per person. This is easy to calculate the savings per year being constant and the time spent in a cohort too.

The consumption rate old is too easy to calculate. It is equal to the number of old people multiplied by what they can consume per year which is equal to the total saved during 45 years divided by 25 years, the time they are still living as old.

You may have copied the ageing chain from the world model and the formulation for the maturations flow are wrong. It is easy to understand. You should take away from the population ageing the people who died during the step which is equal to the population multiplied by the mortality multiplied by the time step. You forgot to multiply by the time step.
This is one of the bugs of the world 3 model. There are probably some others too.

See the thread in this forum

question about the world3-3 model

http://ventanasystems.co.uk/forum/viewt ... orld+model


Regards.

JJ

Thank you so much