Ventana software support forum

You could also formulate it using the XIDZ or ZIDZ macro in vensim

Malli

Hi Bob & JJ, Thanks! JJ - agree with what you are saying. This was a strictly work in progress model, and hence I put the model in before keying in Initial values and doing units consistency. Also, I do not intend to parameterize all values - only very few (maybe 1 or 2). For the rest, I intend ...

I want to optimize a model by making a parameter take different values across time. Essentially, a variable such as 'delay in conversion' could be a parameter. Currently when I run optimization it takes one value which best fits, given the payoff. I feel that making it take different values would gi...

To me, using only measured, hard, variables in a model is equivalent to this sufi tale- A man saw Mulla Nasrudin searching for something on the ground 'What have you lost Mulla?' he asked. 'My key' said the Mulla So the man went down on his knees too, and they both looked for it. After a time, the o...

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 si...

I do not know which version of Ithink you are using. But the DELAY function in ithink is an infinite order (pipeline) delay. The title is misleading. In IThink (atleast on versions 9.0 and below) you have to make the delay structure manually and not use the inbuilt macro for the continuous delays wh...

Hi JJ & Monte, This is with reference to Sterman's book. This weekend, I was revisiting Chapter 17 & chapter 18 - modeling the supply chain after almost 9 months. Working through the chapters once again was fantastic - it felt new (I think it was the Italian writer Calvino who defined a clas...

Hi JJ, Agree with what you are saying in terms of the application of the model to reality. Possibly the best way to get that is go through the apprentice route - where one gets to work with an experienced SD consultant for a number of years and learns the skills. Finding a system dynamicist to work ...

Hi, I have been studying system dynamics for the past 20 months. One of the things I am currently doing is re-doing some the models made in the past by SDers. A good way to do this is to get the MIT D-Memos collection, and work through some of the models in the papers / theses made by SD experts. In...

Many thanks Tony

Malli

Hi,

Am translating a Dynamo model into a vensim one. And ran into this function called SWITCH (a,b,c) in Dynamo - could not decipher waht it means and what would the appropriate way to treat it in Vensim - need help on this

Thanks
Malli

Thanks Bob

Hi, I was using the delay macro in vensim and realized how one needs to be careful with TIMESTEP when using the macro. Instance - suppose its a 6th order delay and I have a total delay time of 1, then the delay in each of the stocks is 1/6. Which meant to me that my earlier DT of 0.25 was not right ...

Thanks Tony. That conceptually makes sense. I will check this with the text.

Malli

Hi, Am reading Tarek Hamid's Software Project Dynamics. While the models are described in DYNAMO and I had no problems in understanding most of the functions - I could not make out what would be the vensim equivalent of dynamo's CLIP (A, B, C, D) . Essentially - there are always 4 vars inside the CL...

thanks bob

Is it possible to convert iThink models to vensim?

Malli

Just another thought on the equilibrium. In my sample model, there is no feedback structure for the inflow. If there is a feedback from the stock to the inflow- then the equilibrium stock can only be achieved by changing one of the time constants also I suppose For instance if h= F/ht then how does ...

My solution is the same as yours. Attaching my sample model For equilibrium in the model- Inflow must be = Outflow(s) Hiring (H) = (F/ AT) + (F/PT) + (F/FT) solving for F F = H(AT*PT*FT) / (PT*FT + AT*FT + AT*PT) Using the parameters assumed in my sample model Initial value of stock F = 2400/26 Agre...

Hi JJ, While what you are saying is right - that in general, any model with a strong negative loop would eventually tend to equilibrium - are there guidelines for initializing a model in equilibrium before testing it with policy inputs? For the simple example of an independent inflow - Little's Law ...

Find attached a model file that shows how smooth works. I have provided two examples -

1. Doing a smooth without using the vensim formula
2. Doing a smooth with the vensim formula

Hope this helps. Would second what JJ has written - do read Sterman on this.

Regards
Malli

Is there any reason why the drop down on time step shows values that are the reciprocal of 2,4,8, 16..128

(Though one can key in a value other than those in the drop down if they choose to)

Malli

Hi Leonardo,

In your model - for the reinforcing loops R1 and R2, there are no stocks. Every loop needs to have atleast one stock . Else the change is instantaneous

I feel you will need atleast one stock in each of those loops

Bob - is my thinking right on this?

Regards
Malli

Many Thanks! That works

Malli

Thanks Bob