circular files; iterating to a "correct" solution

[jefftlewis]

I am working with a circular file. I have been making changes to the file,
and I have noticed at times when i manually calculate that the file iterates
to an obviously "incorrect" solution.

No amount of changing the max no of iterations or the max change seems to
fix the problem. It iterates to the same "incorrect" solution every time.
It is like it is stuck in a rut and can't get out.

I have also noticed that, if I manually calculate each worksheet separately
(that is, hit shift - f9 simulaneously on each worksheet individually, from
left to right) all the worksheets seem to calculate correctly. This leads me
to believe that the file can calculate perfectly fine, and that the issue is
with how excel iterates when it is asked to "globally" iterate (when I just
hit f9 and let excel do its thing).

Am I doing something wrong here? Any know how to fix this?

Regards,
Jeff

 

[Niek Otten]

What is your formula? What are the input values? What result did you expect
and what did you get instead?

Kind regards,

Niek Otten

 

[jefftlewis]

Niek:

Thank you for your reply. It is a fairly involved file, calculating
financial statements for a proposed project. Have you heard of the
phenomenon I describe below? I was wondering if there was a way to somehow
add some flexibility to the iteration process. (I did not create this file
and I hate circular files, I am just stuck with this crummy intentionally
circular file. It would take a ton of effort to rebuilt it and clear it of
the circularity.)

Regards,
Jeff

 

[Niek Otten]

Hi Jeff,

It depends on the functions/formulas.
Personally I would never rely on circular references for iteration; I always
use Goal seek (or, for more complex operations, Solver).
If the function has one or more maximum/minimum the value you start with is
important; iteration will not get you to the other side of a top or bottom.
All that can easily be controlled with Goal seek.
However, if you know a bit about the logic used, you may be able to estimate
a reasonable start value for some of the variables and so influence the
route of iteration.

 

[jefftlewis]

Ok, thanks.

Yeah I don't understand the model real well. I'm going to ask the creator
for help. Perhaps he can suggest what inputs to modify.

The frustrating thing is I am not sure what changes that I am making to the
model are triggering this failure to iterate. And once the model can't
iterate, the file is garbage basically. (I have to revert back to a
previously saved version, and continue working with that.)

So I consider this another fundamental problem with the use of circular
excel files (or circular excel models created by someone other than you, in
which the circularity is not obvious to you).

Thanks, Jeff