Ventana software support forum

hi

im nadia.still new about system dynamic. i just want to ask about lookup function.anybody can give me some briefing about lookup function. and how to decide what range that we have to put.
thank you


Regards,
Nadia

You need to give us a bit more information about what you are trying to do.

For general guidance on lookups, see Chapter 8 of the User's Guide (also in the Help>Vensim Manuals menu). See also http://sysdyn.clexchange.org/sdep/Roadm ... 4653-2.pdf

You can

Actually my topic is about system dynamic model for rice industry.In Malaysia the government supply fertiliser subsidy to the farmers in order to increase the yield which is the unit is in tonne/hactare/year.However the fertiliser subsidy provided by the government is still not enough.The farmers still have to buy the extra fertiliser in order the get the maximum yield. If they only use the fertiliser subsidy provided by the government, they only can get the optimum yield not the maximum amount. But of course, if they add fertiliser more than the recomended rate per hectare, the yield will decline. So my question is, is it possible for me to use lookup function for this situation and how im gonna use it?

Thank you.

Hi Nadia

I do not understand the question. There are contradictions in your report of the problem. What is the optimum yield. Relative to what? You say that 'the farmers still have to buy the extra fertiliser in order to get the maximum yield' and later 'if they add fertiliser more that the reommended rate per hectare, the yield will decline!
As stated the problem has no dynamic in it, it looks more like a simple optimization problem that may be solved by linear or non linear programming.
Regards.
JJ

This sounds like a simple relationship between output and fertilizer so you would use

output per hectare = reference output * effect of fertilizer
effect of fertilize = effect of fertilizer f(fertilizer intensity / reference fertilizer intensity)

with effect of fertilizer f a lookup that goes up for a while and then levels off and maybe even goes down (eg)

effect of fertilizer f
[(0,0)-(3,2)],(0,0.4),(0.246493,0.475248),(0.625251,0.669967),(0.883768,0.861386),(1,1),(1.17836,1.20792),(1.2986,1.33333),(1.41884,1.39274),(1.53908,1.37294),(1.80962,1.27393),(2.23647,1.0297),(3,0.2)

ok.thank you so much..