Good questions.

Using latest and greatest Solver Platform V 6.5.

The Solver objective function is a profit maximisation problem.
Sumproduct(area * profit)

It is changing the area planted of different crops throughout a river basin
to maximise profit. As water is used upstream the salt level increases and
then at each stage

There are 20 stages in the model which water flows through. So as salt
increases yield drops in subsequent stages (Profit here = Yield * Price) and
each of the 12 crops have there own Salinity thresholds levels.

I have the sequential model working (i.e. maximise profit for each stage by
running solver seperately for each stage of the river) and that works fine
(gives the same answer in GAMS).

However, the formula isn't working when I try to solve all stages in a
global solution. It ends up with a value that is less than the sequential
version of the model and according to all economic theroy that just doesn't
happen. The number is also way less than the GAMS output that has been
generated.

The formula I shared determines if the salt level has violated the crops
threshold and if it has then the yield penalty kicks in which ultimatley
affects the profit for the crop.

I have been tearing my hair out with the model for a while until I read that
'IF statements' can cause problems with the solver.

I tried the trick of checking the model for structure, checked the model for
Transformed structure, selected the solve transformed problem and then used
solve the model and it told me the model was to big. (This is due to the
economic state contingency methodology we are useing as their are three of
the 20*12 tables to determine the effect of salt for 3 possible states of
nature).

Any suggestions would be appreciated.



"Tushar Mehta" <tm_200310@tushar_hyphen_mehta_dot_see_oh_em wrote in
message om...
The IF statement actually is a MAX(x-threshold,x) not that that helps
with Solver.

What does it mean the IF statement is "bad?"

What is the Solver model? The objective function? Are you maximizing
it? Minimizing it? What are the decision variables?

The formula you shared...what is it? How does it fit into the Solver
model.

--
Regards,

Tushar Mehta
www.tushar-mehta.com
Multi-disciplinary business expertise
+ Technology skills
= Optimal solution to your business problem
Recipient Microsoft MVP award 2000-2005