How to model 2 stocks feeding a 3rd one through a single valve?

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Vensim version: PLE

How to model 2 stocks feeding a 3rd one through a single valve?

Postby bffcosta » Mon Aug 08, 2016 10:53 pm

Dear Messrs.

good evening and best wishes.

I am trying to build a model following the logic embedded on paper System dynamics applied to project management: a survey, assessment, and directions for future research by Lyneis and Ford. In their paper, they suggest that two stocks (Original Work to Do and Rework to Do) are feeding a third one (Work Done) using a single rate named Progress (please see the enclosed document). I would appreciate it if someone could help me by telling how to create this structure using Vensim PLE (I mean manipulating the software to connect the first two stocks to the third one, using this very single valve/rate).

Kind regards,

original work and rework feeding work done.jpg
original work and rework feeding work done.jpg (297.35 KiB) Viewed 744 times

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Re: How to model 2 stocks feeding a 3rd one through a single valve?

Postby Administrator » Wed Aug 17, 2016 6:59 am

I think what you are seeing in the diagram is a trick of the graphics.

Without seeing the model I cannot be sure. My guess is that "Progress" drains "Original work to do", it is not connected in any way to "Rework to do". "Error generation" drains "Rework to do", again not connected to "Original work to do". The pipes are drawn on top of each other giving the impression that they are all connected.

Unfortunately the paper does not include any of the equations for the parameters, so I cannot be absolutely sure.

Personally, I would avoid that structure anyway, I find it slightly confusing. How can "progress" deplete "original work to do" and "rework to do"? Logically it would deplete "rework to do" first, and then any remaining "original work to do" after.

I'd start with a stock that is "work to do". The initial value is "original work to do", and drain this using "progress". Use "Progress" to calculate rework and place this back into "Work to do".
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