Ventana software support forum

hi,

I have two variables dependent from each other (a=10*b and b=10*a). altough I gave them an initial value in the function ACTIVE Initial, I get an error message. How can I solve this equations without using a level-function, wehre the values are accumulated?

thanks for your help... greets patricia

If a=b*10 and b=10*a then a and b must both be 0 which means they are actually the same.

More generally if you want to have sets of simultaneous equations the best thing to do is to solve them out explicitly in terms of other variables. The SIMULTANEOUS function and the FIND ZERO function can be used with care in stituations like this but are intended only for versy specialized situations.

Please note that simultaneous equations do not exist in physical or information processes except at the quantum level. If you find that you are trying to use a simultaneous equation you probably have a formulation problem.

thank you... I think I have to concretize my problem: It is a model about the market of the generics. The market share from a generic company is dependent from the behavior of the physicians. This behavior is dependant of the actual market share and social effects between the physicians, which are again (with delay) dependent from the already reached behavior. Because these variables aren't stocks, I won't accumulate. How can I make them dependent from each other?

Hi

You must define if your variables, are stocks or auxiliary variables.
The market share is obviously a stock (a situation) and some other variables may too be.
These stocks will help you model the part of the model that is dependant of the past.
It would be too a good idea to send a very simplified model of your problem as an example.
Regards.
J.J. Laublé

hi and thank you ...
So I can't use information from a earlier timestep without implementing stocks?
I just want to have the value and not the integral as input for the next step. I have attached a part of my model (see 2nd view). My problems are all the loops! (the equations arn't right yet.)
greets patricia:)

Hi

Here is attached a simple model, that explains how you can keep an information from one step to the other.
One observation, you should simplify your model, and check ir for errors and unit errors.
There are too variables with no values that are not visible on the sketch.
That does not ease the understanding of the model.
It is simple to send an extremely simplified model, that resumes easily your problem.
You should study a bit too , the Vensim user guide, which is the best book I know for a beginner.
Regards.
J.J. Laublé

hi,

thanks a lot - this problem I could solve now! (sorry for the bad model, but I thought you could see the model with the arrows and so on).
I have lots of other problems...so I will be on the forum the next few days
Do you know a exemple for market share with more than two competitors?

thanks and greets patricia

Hi

Try the SD mailing list, where you can ask the question.
You can subscribe easily on the Vensim Web site.
I am currently making a model optimizing price setting and investments, but I have not so far introduced the competition, as I work step by step.
Businees Dynamics from Sterman and Strategy dynamics from Kim Warren are too useful reference books.
Regards.
J.J. Laublé

You can also try searching the SD mailing list archive,

http://www.ventanasystems.co.uk/forum/v ... m.php?f=34

I need to update it with around three weeks of posts, but there is a lot of information there as well.

Tony.

hi everybody,

thanks for the helpful answers...

Is there another possibility to let a variable x start at a special time without using another variable y before with the function STEP? Because if I implement an extra variable y with STEP, the variable x starts one timestep later. So do I have tho substrate this timestep?

Another question just to be sure:-): With Vensim PLE plus I can't operate with vectors, do I?

regards, patricia

Will an "IF THEN ELSE" do?

PLE Plus does not contain subscript functionality. Have a look at
http://www.vensim.com/comparison.html, that lists all the functionality of the different versions.

Tony.

Hi

You need to have Vensim Pro to use vectors or arrays.
A variable will have a value from the start of the simulation or the value :NA: which means no value.
I you want the value of a to have no value until the time begintime you will define the variable a = if then else (time < begintime,:NA:,the function from what will depend A or a constant)
Hope that helps, otherwile be more precise.
Regards.
J.J. Laublé

hi and again thank you!

So easy with if then... (I knew this function but did search for a time function)

For avoiding the slow increase of a variable from the start time (until start time the value =0) to its value at the next time step, the only way is to choose the time step as small as possible, isnt'it?

Greets patricia

the variation of a variable from one step to the other will be slowed or accelerated with the decrease or increase of the time step indeed.
If you are running at 20 kilometers an hour, the length of the journey in a millisecond will be shorter than in an hour.
But it will not change the overall speed.
So to avoid the slow increase of a variable, you must on the contrary increase the time step... and study the
user guide.
Regards.
J.J. Laublé