Ventana software support forum

I treid to get this model working, if any body has any ideas about how to get this actual model working I would be extremely greatful.
Julian

Hi Julian,

Your model has several inconsistencies.

The key reason your model is not running is because of the loop involving the variables 'Amount Advertised' and 'Distance from Advertising Saturation Threshold'. Every loop in a SD model needs to have atleast one stock else it becomes a system of simultaneous equations - all calculations happen instantaneously resulting in the error. In this loop there is no stock.

But besides this error, there are several other things you might want to correct-

1. There are no units for your variables. Putting down units will definitely help clarify your thoughts better. For instance, Revenue = Amount Sold, and Units Produced = Amount Sold. There is a unit inconsistency here - Revenue could be in dollars and units produced in 'units'.

2. The loop involving 'Amount Sold' and 'Distance from Market Saturation Threshold' is faulty. You have 2 stocks connected to each other without flows. You might want to use the standard notations for stocks (the box variables) to retain clarity

3. Your timestep is 1 in the model settings. Is it appropriate is something you might want to check

There are several excellent resources on system dynamics. One option would be to get John Sterman's book. Several free resources are available on the net. You can check out -

1. The introductory materials listing in the system dynamics society's homepage. These are the road map series and is a good place to start
http://systemdynamics.org/other_resourc ... erWebSites

2. You could also check of Craig Kirkwood's page at ASU. It has got some excellent tutorials on system dynamics as well as vensim.
http://www.public.asu.edu/~kirkwood/sysdyn/SDRes.htm

3. The vensim manuals also give a good intro to SD modeling

Regards
Malli

[Edited on 1-18-2008 by malli]

Hi Julian

You already asked the same question, in the same forum, thread conderning simultaneous equations, the 25th of december 2007.
I posted two optional models.
if you have not well understood the models joined, it would be better for you to write down in plain english what you are trying to do. Until you have done that you will have no definition of what you are trying to do. It is like starting on a trip without knowing the destination!
A subsidiary question:
Are you a student or from the academic world, or are you a
practitioner?
Regards.
JJ

[Edited on 18-1-2008 by LAUJJL]

[Edited on 18-1-2008 by LAUJJL]