A problem with the simulation

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Abhyank Srinet
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Joined: Mon Jun 22, 2015 4:07 am
Location: Paris
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A problem with the simulation

Post by Abhyank Srinet » Thu Jun 25, 2015 3:43 am

Hi Guys,

Well i am a starter with SD and am slowly building my knowledge via self learning.
I was trying to build a simple model on Job migration, but am quite baffled as for some reason the No. of workers is not reaching the goal of 5000 set, and is becoming constant at 4444.

Any help in figuring this problem out will be deeply appreciated.

Thanks for the help.
Regards,
Abhyank

P.S- I am attaching the file as an attachment below, i used vensim to build it
Attachments
worker migration.mdl
(2.6 KiB) Downloaded 298 times

Rod MacDonald
Posts: 1
Joined: Tue Feb 17, 2009 3:47 pm

Re: A problem with the simulation

Post by Rod MacDonald » Thu Jun 25, 2015 9:52 am

Hello Abhyank,

What you have is a steady state error. Basically, your formulation does not take into consideration the Retirement Rate in determining the Migration Rate In. If you set the Retiring Constant to zero and simulation your model you will see that you achieve a steady state equilibrium with the desired 5,000 workers and that not taking into consideration the Retirement Rate is the problem.

I would attempt to describe steady state error in more detail, but the best description and solution to this problem can be found in Sterman (2000) pages 671 to 672. Sterman also has a very nice Inventory-Workforce model (see pages 768 – 769) that contains a labor sector that explicitly takes into consideration vacancies. You may find some of the formulations and structure helpful for what you are doing.

Good luck,
Rod MacDonald
Albany

Citation
Sterman, J. D. (2000). Business Dynamics: Systems Thinking and Modeling for a Complex World. Boston, MA, USA: Irwin McGraw-Hill.

Jean-Jacques Lauble
Posts: 74
Joined: Fri Aug 23, 2013 3:49 pm

Re: A problem with the simulation

Post by Jean-Jacques Lauble » Thu Jun 25, 2015 11:43 am

Hi

Joined a model modified that will reach the goal by either increasing the final time if the capacity of migration in is limited or increasing the capacity of migration in or both.
You can do this with the two sliders and see the result in the graph.

Regards.

JJ
Attachments
worker%20migration[2].mdl
(3.35 KiB) Downloaded 288 times

Richard Dudley
Posts: 65
Joined: Sun Jan 11, 2009 1:17 pm
Location: Etna, New York, USA
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Re: A problem with the simulation

Post by Richard Dudley » Thu Jun 25, 2015 1:11 pm

I assume you have had several replies to this question.

Actually your model is correct IF the personnel department does NOT take into account the retiring workers when they are making hiring decisions.

If you believe that the personnel dept does recognize retiring workers then you need to add hiring of these replacements to the model. Typically this is done via some function of the outflow usually a smooth of what you call the retirement rate. This would give you something like "expected retirement rate"

This problem is referred to as a "steady state error" See Sterman 2000 Section 17.2.2.

One option is in the attached model.
worker migration 2.mdl
(3.47 KiB) Downloaded 308 times

Abhyank Srinet
Posts: 4
Joined: Mon Jun 22, 2015 4:07 am
Location: Paris
Contact:

Re: A problem with the simulation

Post by Abhyank Srinet » Fri Jun 26, 2015 3:13 am

Thank you for the replies!
It really helped solve my doubts :)
You guys are great.

Regards,
Abhyank

James Thompson
Posts: 44
Joined: Sun Jan 11, 2009 11:41 am
Location: Farmington, CT USA

Re: A problem with the simulation

Post by James Thompson » Fri Jun 26, 2015 8:12 am

You might find answers in pieces in the article:
Ansah, J. P., Riley, C. M., Thompson, J. P., & Matchar, D. B. (2015). The impact of population dynamics and foreign labour policy on dependency: the case of Singapore. Journal of Population Research, 32, 115-138.

We document the model, including dynamic equilibrium.
Good luck,
Jim
Jim Thompson
jimthompson@live.com
jpthompson@wpi.edu
+1 860.796.2428

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