Ventana software support forum

Hi there!

I have a question regarding putting in variable values for each time step for a supply chain simulation over the course of one year. Basically I have 27 countries with production value as a variable, export and import as inflow and outflow with each other country varying each month of the year. This model will then be used in a sensitivity analysis where I will alter the input factors - but for that I need a sufficient model than runs smoothly.

Any help is appreciated! I am a complete VENSIM PLE beginner and very happy for any help I can get!

Looking forward to your responses. Thank you in advance!

Greetings from Germany, Marie

I don't understand what you are asking for help with.

If you have 27 countries, you would really be better off using Vensim Pro as it allows subscripts/arrays which will allow you to model all 27 countries at the same time with a single model structure.

Vensim PLE also does not have the ability to do sensitivity analysis.

Thank you for your quick reply.

What I want to simulate is a trade relationship so let's say Austria will get an inflow of product from Germany which differs every month (January=0, February=200, March=300, May=176 and so on). And then I wish to alter the input factors and analyse the overall reaction of the whole model. I am not sure if I have access to the Pro Version through my university - so the PLE Version is useless for what I want to do?

PLE is designed for learning about dynamics, not for detail-oriented work. However, if you can simulate the 27 countries independently, you might make it work. You could put the demand stream in a lookup (as a function of time). Then you could load different streams from changes files (.cin) for testing.

So you mean integrate a Lookup table with the values for the different imports in the different time steps? Why would I need to model each country independently though?

Correct.

I guess you don't have to model the countries independently, but copy-pasting the structure 27 times to do them together without arrays sounds painful.

Okay thank you so far! So in a Lookup table, can I not only tell VENSIM the different values, but also the origin of each value? In order to actually model a trade relationship

A lookup has one input (e.g., time in this case) and one output.

You can create arrays of lookups, where the subscript elements represent other attributes (country, commodity, etc.).

And there is no other way to work around that using VENSIM PLE?

In PLE you have 2 options:

- Build a generic model of a single country, and then load different values of parameters and the demand lookup from a changes file, with 27 distinct copies of the file.

or

- Build the model for one country, then copy-paste it 26 times. You could use generic lookups for the initial instance, but then modify the data using a change file (you'd only need one, which would be a little more convenient).

If you need a 27 x 27 trade matrix, I think these approaches are impractical. It would be easy to handle that with arrays in Pro/DSS.

Thank you! Well but what I need is to also analyse the relationship between the countries. So I need them all in one model. I will not be able to finance the pro version, is there not a free trial version of the Pro?

There are deep discounts for academic and public research purposes. You can shoot us an email at vensim at vensim.com for a quote. Possibly there's an unused copy at your institution?