Vensim constant behavior

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anca91
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Vensim constant behavior

Post by anca91 »

Hello everyone, sorry if this topic could be banal. I have the following model and I try to verify his behavior. Unfortunately, when I start the simulation I get a constant trend, when I expect a decrease in the number of uninfected and an increase in the number of those infected. You could kindly help me. :oops:
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Administrator
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Re: Vensim constant behavior

Post by Administrator »

Try changing
rate of plant infections = Plant Infected/LATENCY TIME
to
rate of plant infections = Plant Not Infected/LATENCY TIME

At t = 0, the number of plants infected is zero. So rate of plant infections is also zero.
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anca91
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Re: Vensim constant behavior

Post by anca91 »

thanks for the reply, I tried to do what you say. but I get this answer: :cry:

ERROR: Floating point error computing - rate of plant infections - at time = 34.000000.
Trying to save the results anyway
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Re: Vensim constant behavior

Post by Administrator »

Have a look at a table of the values for rate of plant infections and see what is happening.
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anca91
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Re: Vensim constant behavior

Post by anca91 »

I can not find a meaning
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Re: Vensim constant behavior

Post by Administrator »

One problem is that your model is running for 100 years with a time step of one year. This needs to change, the time step should be 1/4 of the smallest delay in your model (I think this is 0.08333, the "latency time").

What is the "latency time"? I know it's value is one month, but what actually is it?
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anca91
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Re: Vensim constant behavior

Post by anca91 »

It is the time it takes to understand how much time there is between when a plant shows the disease and when it takes the infection. :roll:
tomfid
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Re: Vensim constant behavior

Post by tomfid »

The model will not generally be stable unless the time step is shorter than the shortest delay. Powers of 2 are generally better, so perhaps .03125 would be a good start (you can set this via the dropdown menu at Model>Settings>Time Bounds.
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