Descriptive variables definition in Vensim

[Hossein]

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,

[Administrator]

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.

[Hossein]

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.

[Administrator]

Yes, look in the documentation for VECTOR LOOKUP, this will let you specify individual points in a lookup table that you can calibrate.

[tomfid]

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.