Dear Tom,
I want to model the safety issues in mining equipment usage in Vensim. Before it, I have just used from parameters that they have constant values and variables can be precisely determined by equations . However, in this case, I just have some descriptive issues which there is no equation and data about them. For instance, I know if the "Traffic of trucks" increases, the "safety" will decrease but, there is no equation between these two items. My question is that who can I deal with such problems in Vensim?
Sincerely yours,
Descriptive variables definition in Vensim
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Re: Descriptive variables definition in Vensim
Start with an estimate, and then try and find some data and calibrate the model against that data to refine the estimate of the unknown constant.
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Re: Descriptive variables definition in Vensim
Thanks for your reply.
But is it possible to use LOOKUP (or any other function) for such problems? because we do not use equations in LOOKUP but just define a function.
But is it possible to use LOOKUP (or any other function) for such problems? because we do not use equations in LOOKUP but just define a function.
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Re: Descriptive variables definition in Vensim
Yes, look in the documentation for VECTOR LOOKUP, this will let you specify individual points in a lookup table that you can calibrate.
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Re: Descriptive variables definition in Vensim
VECTOR LOOKUP works well, but I try to avoid it, because it's hard to enforce constraints that lead to a sensible shape. You might get a nonmonotonic or irregular result.
I usually try to find a flexible analytic function first. For example, if you now use:
safety = normal safety * safety lookup( traffic/normal traffic )
you could try something like:
safety = normal safety * 1/(1 + a * ( traffic/normal traffic ) ^ b)
this guarantees a roughly s-shaped decline in safety with increasing traffic, and has only a few parameters.
I usually try to find a flexible analytic function first. For example, if you now use:
safety = normal safety * safety lookup( traffic/normal traffic )
you could try something like:
safety = normal safety * 1/(1 + a * ( traffic/normal traffic ) ^ b)
this guarantees a roughly s-shaped decline in safety with increasing traffic, and has only a few parameters.
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