Question

Sigmoid Formula For "S" Curve

Asked by: DougDodge

I am looking for a formula for excel to produce an "S" curve.  It will be used for Project Control purposes in developing proposals.  I wish to give it a number of variables:
1) Start
2) End
3) Maximum value
4) 1st point with a percentage to calculate arc.
5) 2nd point with a percentage to calculate arc.
6) Number of period for duration.

Although this is my initial thoughts on this, if anyone has a better way to calculate it to go into a formula they might have, I would be open to review it as well.

This could be in a formula for a cell entry, or in VBA.

Thanks in advance.....

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Asked On
2008-11-20 at 17:49:40ID23923781
Tags

Microsoft

,

Excel

,

2003

Topic

Microsoft Excel Spreadsheet Software

Participating Experts
2
Points
250
Comments
5

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Answers

 

by: patrickabPosted on 2008-11-21 at 11:53:19ID: 23016355

DougDodge,

Please specify the exact Sigmoid formula you are intending to use. They are often of the type:

f(x) = x/(1+x^2)^0.5

An example of this curve is in the attached file.

Patrick

 

by: DougDodgePosted on 2008-11-23 at 14:35:01ID: 23026132

Patrick:
I have attached an example of what I am working on.  The problem comes when the "Engineering" curve is to start from (0) zero, but actually atarts at 1.6%

The rate of accent can be adjusted, but not quite the way I would like.  I guess some more tweaking of the formula's are required.

I'm not quite sure where to go from here.

Regards,

 

by: patrickabPosted on 2008-11-24 at 15:31:05ID: 23031448

DougDodge,

This is a fudge but it does start the curve at 0 and end it at 100:

=$G$11/(1+EXP(-$H$11*(DU$2:IA$2-$I$11)))-(((125-COLUMN())/110)*0.016)

It's in the attached file.

Patrick

 

by: WeirdunPosted on 2008-11-25 at 23:09:27ID: 23039971

You can create the function in vba which you can call from your worksheet  like so.

Function Sigmoid(t)
Sigmoid = 1 / (1 + Exp(-t))
End Function

 

by: DougDodgePosted on 2009-04-13 at 10:58:49ID: 31518986

This worked, sort of.  I found something from a mechanical eng'g tech that works better.

20120131-EE-VQP-002

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