Ventana software support forum

Hi,
Im Nadia. Master's student from Malaysia. Actually sd is really new in Malaysia. No expertise yet in Malaysia. I already try build the model but this can not be run. Is it possible if I attached the model and get the comment from you? This problem is regarding to the rice stock.
The inflow for the rice stock in Malaysia is come from local rice production. Since the paddy production in Malaysia not enough to meet the demand, therefore, we need to import. However the import will not included to the rice stock. However, the government also imposed import quota (import restriction) at certain level. If the import quota is less than the desired import, therefore the government has to release certain amount of rice stock in order to ensure demand=supply.
My problem is i dont know how to convert this situation to stock and flow diagram. And most of the papers included the import in the rice stock. Which is rice stock=import+rice production-demand. But this situation does not occur in Malaysia. In malaysia, import will not included in the rice stock. So how I can build stock and flow diagram for this situation?

Thank you.

Regards,
Nadia

Post the model and I'm sure someone will help.

The rice stock model that cannot be simulated due to simultaneous equation as attached.

Do you understand why there is a simultaneous equations error detected?
JJ

hi,
Yes, because there must be at least one stock for each feedback loop right?

Yes and no.

One other way to deal with a simultaneous error is to use a simultaneous function or a find zero function. See the explanations of these functions in the help. It is simpler to use a stock in the loop to allow the variable in question to change value at each time step than using these tricky functions. The solution for you is then to add a stock somewhere in the loop. You have then to think a bit about your problem to find the solution.
If you think that the variable adjustment takes too much time (in your case a term) you can decrease the time step.
If after a good while, you still cannot find the solution, then simplify your model to the utmost and try again. It will simplify your problem too and if you still cannot find the solution, you will ask to the forum the solution to that simplified problem and it will make it easier for somebody to help you!
Regards.
JJ

[Edited on 9-11-2010 by LAUJJL]

Ok. thank you so much for your suggestion.I'll try to do it again.

Regards,
Nadia

Don't use the simultaneous function or a find zero function here (they are not suitable).

What you need to do is treat imported rice as a separate stock. Start by duplicating the structure you have for Rice Stock (so you will have "imported rice stock", which is increased by "import" and decreased by the demand). Get this working and then you can figure out how to link the two parts of the model.

What you need to do is treat imported rice as a separate input to rice stock. Look at the attached model.

[Edited on 9-11-2010 by Rafael]

[Edited on 9-11-2010 by Rafael]

[Edited on 9-11-2010 by Rafael]

[Edited on 9-11-2010 by Rafael]

ok..thank you so much.

Regards,
Nadia