Ventana software support forum

Am I allowed to post questions about iThink here? I've been working with it about 1 month, and I'm still a little stalled on a few basic very simple questions.

1. What is the best way to represent a stock capacity (as in "holds 3 gallons and more won't fit"?) An "if (<= 20) then X else y" statement works, but is that the best way? A "reservoir" doesn't seem to have a capacity limit checkbox, but a "conveyer", and I think an "oven" do....but the flows following them don't permit me to put a "flow per time unit".

2. I still get hung up on the math. This is silly, but does anyone know of any good resources for the most basic-basic math as it relates to modeling....including simple use of percents and fractions or differences...and when to use what? For instance, if "Joe" is motived to exercise by the difference in weight lost, then is that best represented as a difference...or a fraction between beginning weight and weight now...or should I just use a graphical function. I see percents and fractions used lots but I always wonder if I'm choosing what's best.

Thanks folks,

BL

No problem if you want to post iThink questions here. Hopefully one of the regular readers is familiar with it and can help you a little.

Tony.

Hi Barry.

I do not know if you will find many people to help you on I Think strictly speaking.
But this forum is working very well and people can ask any questions they want.
One thing that you could do is to download a version of Vensim PLE that is free for learning and
that can do already a lot. You can save your work and are only limited by some functionality that is only useful
after a long time of practice.
If you have already bought I Think, working in parallel with both software at least for a while is not a waste of time because in SD the quality of thinking is more important than the software.
As to your second problem, you can choose any solution.
First solution: you can decide that any time the difference between the actual weight and the intended weight say 80 kgs is more than 2 kgs, it is necessary to make some exercise.
You can too decide that any time the ratio between the actual weight and the intended weight is more than 1.025 you will make some exercise. In fact the result will be exactly the same.
You can decide too to make the quantity of exercise depend on the gap between the actual and the intended weight
whether expressed by a difference or a ratio.
You will then build a table function that will tell the amount of exercise corresponding to a certain gap.
This method is probably the more precise, but is a bit more complicated.
In SD the more complicated is often not the more efficient.
Regards.
JJ

Hey, thanks much for that. The multiple ways of addressing the same problem was very insightful. I'm studying the books pretty hard, and stalling on basic questions. Thanks again.