How to model random delay within min/max?

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airfix
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Joined: Tue Dec 18, 2012 10:04 am
Vensim version: PLE

How to model random delay within min/max?

Post by airfix »

I am using the trial PLE version to assess whether it suits my needs (and whether I can drive it).

Starting with a simple flow of petroleum cargo imports, storage, despatch and replenishment, with some stochastic variables. I have yet to find how to introduce a random delay to model variable vessel discharge times (eg vessel can be 1 day early or up to 2 days late due to berth congestion etc).

I can see fixed period delays (which I will use for order to on water time) but nothing that gives, say, a normally distributed random delay.

In another tool I vaguely remember using a random number generation step and then, separately, an equation that draws on that random number to effect a delay.

Any suggestions about how to model the delay stochastically?
Administrator
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Re: How to model random delay within min/max?

Post by Administrator »

In another tool I vaguely remember using a random number generation step and then, separately, an equation that draws on that random number to effect a delay.
That is exactly what you do in Vensim as well.

First generate the random number
http://www.vensim.com/documentation/ind ... random.htm

Then use delay material
http://www.vensim.com/documentation/ind ... terial.htm
Advice to posters seeking help (it really helps us to help you)
http://www.ventanasystems.co.uk/forum/v ... f=2&t=4391

Units are important!
http://www.bbc.co.uk/news/magazine-27509559
airfix
Junior Member
Posts: 16
Joined: Tue Dec 18, 2012 10:04 am
Vensim version: PLE

Re: How to model random delay within min/max?

Post by airfix »

Thanks for the tips.

WIth Delay Material unavailable in PLE (or PLE Plus) I am looking for a workaround.
LAUJJL
Senior Member
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Vensim version: DSS

Re: How to model random delay within min/max?

Post by LAUJJL »

To have a variable delay you need the delay material function not available in the PLE version.
The solution is to use several delay fixed variables with a different delay each, and then to choose which delay to use depending on
a random function.

Regards.

JJ
airfix
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Joined: Tue Dec 18, 2012 10:04 am
Vensim version: PLE

Re: How to model random delay within min/max?

Post by airfix »

Thanks JJ ... Focusing on bigger alligators in my model, but will come back to explore your suggestion.

Michael
tomfid
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Re: How to model random delay within min/max?

Post by tomfid »

A normally distributed delay is not physically possible, because it includes negative arrival times. Practically, this may not be an issue, if the mean is large such that the probability of a negative arrival time is vanishingly small. But generally, you want something like a Poisson process (RANDOM POISSON) that has some grounding in theory.

There's an example here:
http://models.metasd.com/rental-car-sto ... -dynamics/
tomfid
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Re: How to model random delay within min/max?

Post by tomfid »

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