Help in solving a problem

[anjomshoae]

Dear all,


I am developing a model for my research, in my model I have a stock name "Unsatisfied Demand". The value of stock becomes negative, which is not rational. What I want to model is the backlog of people who did not receieve aid. Can anyone check my model and help me to improve it. Any suggestion?

I attached my model to this email.
Thank you for your time in advance.

Regards

[Administrator]

I've fixed that problem with the level going negative. You were removing more "unsatisfied demand" than was available.

You should put units into your model as well. It helps others work out what the model is supposed to be doing. For example, what is "unsatisfied demand" actually measuring?

[anjomshoae]

Thank you for your help. I appreciate it.

I included units in the model and tried to fix the unit errors. I fixed a few of them. But still, only two of errors are persisting. Specifically the error related to evacuation. I don't know how to fix them. can you please help.

please not that I attached the model to this post.

Thank you.
Best,

[Administrator]

My guess is that "evacuation rate" should be a percent/month. But this is just a guess as "Lookup of evacuation rate" and "Flood Risk" are both dimensionless so there is no way I can be sure what they are measuring.

[anjomshoae]

My guess is that "evacuation rate" should be a percent/month. But this is just a guess as "Lookup of evacuation rate" and "Flood Risk" are both dimensionless so there is no way I can be sure what they are measuring.

So I tried to fix the errors and I think wrongly I have used two table function. I removed one of them. Now the model structure is changed. I am not really sure that the model is correct now. would you please have look at it and let me know your thoughts. I do appreciate it.

[Administrator]

It's difficult for us to take a look at a model when we know nothing about what it is supposed to be doing.

So if you can let us know why you think the model is incorrect, maybe we can advise you.

I did notice that "UNHRD Workforce" starts negative. Is this correct? And "Incountry Inventory UNHRD", what should happen here?

[anjomshoae]

It's difficult for us to take a look at a model when we know nothing about what it is supposed to be doing.

So if you can let us know why you think the model is incorrect, maybe we can advise you.

I did notice that "UNHRD Workforce" starts negative. Is this correct? And "Incountry Inventory UNHRD", what should happen here?

Thank you again.

I think as you said the model is not showing the reality. Because the inventory gets negative and workforce starts negative. The model structure is taken from the Vensim help chapter 2, modeling guide-- concept with example. I have read this chapter and I borrowed the model from there, however it is mentioned in the "Extensions and Exercises" that " If you look at the model structure we have developed you will see that it fails the Reality Check "Without Inventory we can't ship." Try to develop a structure to correct this flaw. You will need to create an explicit shipment variable and use a nonlinear function (Lookups are often used here) to constrain shipments."

I don't know how I can correct this problem and because the help of Vensim is not comprehensive I don't have access to better resources. how do you think I can improve it? I do appreciate your help.

Best Regards

[Administrator]

It's usually straightforward to stop a level going negative. The following thread has a lot of useful information.
http://www.ventanasystems.co.uk/forum/v ... f=2&t=6186

[anjomshoae]

It's usually straightforward to stop a level going negative. The following thread has a lot of useful information.
http://www.ventanasystems.co.uk/forum/v ... f=2&t=6186

I read the thread and I tried to change the structure of model accordingly (colour changed to red for better identification), but the model behavior got even worse than before. Is the way I implement the change correct ? if it is correct then there might be a problem somewhere else in the model. any hint or suggestion ?