Simultaneous equation

[Anvar]

Dear members, hello:

I was taking system dynamics course where I got assignment. And I have some difficulties in it. Actually i am new in vensim, so I will be grateful if you can help me with your advices to avoid the error I am getting in my model and your comments.
I am modeling the general flow of manufacturing processes of some good (long-run). First, manfacturer decides how much to produce after measuring demand for it (it should within the budget - i am getting error here). Then by applying necessary amount of raw materials, labor and equipment (depending on the costs of labor and equipment the manufacturer may reduce for ex. number of equipment and hire more labor). My aim is to show how profit margin of manufacturer will change if to change demand, costs and so on.
I've attached what I tried to do. I didn't add stock and flows - because the values may change to negative also
Thank you very much in advance.

[LAUJJL]

Hi Anvar

Yous should first correct dimension errors, by putting the right unit dimension for each equation. Good practice.

You should too make the sketch more readable, by putting for example levels within a box. It is too a good idea to materialize the flows either by a special double arrow provided by the vensim language or by distinguishing the influence arrow when it is a flow from the other ones that are mere informations. A difficult to read sketch is not helpful and does not convey the right information and does not help anybody to give you advices.
A simultaneous error happens when a variable is influencing itself within the same time step. The solution is either to make the variable influence itself but within the next time step by making the loop run through a level, or to use a simultaneous or find zero function that will allow the different values of the same variable to converge to a unique value. See the reference guide for simultaneous function and find zero function.
Regards.
JJ

[bob@vensim.com]

If you are taking a system dynamics course you should stay away from SIMULTANEOUS or FIND ZERO. You need to identify the Levels for your model - for example Labor is a level increased by hiring and decreased by attrition and layoffs. If you have a textbook or recommended readings for the course I would start with those.

[LAUJJL]

Simultaneous or find zero functions are tricky and certainly not for beginners , I used them sometimes ago but I use them only if there is no other alternatives.
JJ

[Anvar]

Dear LAUJJL and Bob,

Thank you very much for such quick response.

I will try to use the idea about the influencing the variable in different time steps. I hope it will work.

Thank you once again.