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## Power and Exp Regression?

Why are guess values needed in MathCAD for power and exponential regressions? Excel doesn't need them. Attached is a mathcad and excel document that gets me nothing but frustrated. All I have is x and y values that I am passing into both MathCAD and excel. MathCAD can't find the a,b and c values to satisfy the regressions, even when I give it the same ones I found using Excel. I even graphed the excel values in Mathcad and they worked. Why doesn't Mathcad find the coefficient values of the regression?

Matt Pittard
16 REPLIES 16

## Power and Exp Regression?

1. The function expfit, pwrfit etc solve *nonlinear* task and cann return two or more answer. Thay need guess values.
2. Statistic in Excel is very bad.

Valery Ochkov
http://twt.mpei.ac.ru/ochkov

## Power and Exp Regression?

Yeah, Excel has terrible Statisticial regression analysis. They don't print out nearly enough significant figs. My problem was that MathCAD wasn't finding one of the nonlinear solutions.

Matt

## Power and Exp Regression?

Origin is superb on some aspects.
It was no sweat fitting your data.
By the way, some of the data are in error. In such case, it's good practice to eliminate the bad ones. If your project was " real life " then use it. I think this the true approximation .

## Power and Exp Regression?

Origin is superb on some aspects.
It was no sweat fitting your data.
By the way, some of the data are in error. In such case, it's good practice to eliminate the bad ones. If your project was " real life " then use it. I think this is the true approximation .

## Power and Exp Regression?

Try these values

Chi2 R2
---------------------
1.43763 0.99916

A1 = 2139.745
t1 = 1.2507
A2 = 171.36
t2 = 3.832

## Regression - visual control of guess values

Hi! See:

PS I think that Excel has guess values too - zeros.
Valery Ochkov
http://twt.mpei.ac.ru/ochkov

## Regression - visual control of guess values

The approximation I gave yesterday, was extracted using Origin. This software does not require initial guess values. By default it varies each parameter. But the number of iterates may be specified. The first approximation was with 1 iterate, the second one used 10 iterates.

## Regression - visual control of guess values

Hi,

sorry to disagree about Origin, it too uses guess-values!

It obviously uses good default-guess-values, that you don�t see, when just fitting a curve through data using the menu for the simple fitting.

But if you go into NLSF (non-linear fitting) and check "more/advanced", you will see the according equations, their parameters and start-values! (Bad guess, bad fit, that easy).

Origin is really great for displaying and analyzing data!

Hubert

## Regression - visual control of guess values

You are right Hubert. Since I have Mathcad, I rarely use Origin. You like it too ! So nice graphs. Combining Origin and Mathcad, would be near perfect.
I tried Origin for one of my favorite rational fraction approximation, then it has no guess values because there is no model. It comes close to exact answer, just by itself, and there are 3 coefficients. I do the same with Mathcad, but in steps. It could be programmed in Mathcad, but I'm not skilled at that.
As a matter of facts, for certain type of curves, I can generate an excellent P(x)/Q(x), in simple steps.
Why these big guns: Mathcad, Mathematica, Mathlab... are so poor about approximating functions ???
Can you imagine, none of them included the Thi�l� Continued fraction...F.
If you read me, Danish Collab, I will make a file for you. Because Thi�l� is Danish. And such a great tool, you must have it.
I have many sheets, using Thi�l�. Yes Sir !

## Regression - visual control of guess values

Guys--where do I get a copy of this "ORIGIN 6.0"--how much does it cost?

Matt

## Regression - visual control of guess values

I copied the page that remained in my mail box.
In 6.1, you will find added functionalities. As such, it's the same software.
Their forum is well, well, well supported.
Superb companionship with Excel. Origin is a spread sheet type too.
But not very convenient writing equations in script format.
I have not tried it with Mathematica.
Origin is a very interesting gear.
I believe < \$ 1000.
I think their latest link is:
http://www.origin.lab

Let me know if you have difficulties.

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## Regression - visual control of guess values

Sorry. But it is the chat about Mathcad. I know a lot of software for regression analyze (best than Excel, Origin etc). I can tell about this software, but thru e-mail.
Valery Ochkov
http://twt.mpei.ac.ru/ochkov

## Regression - visual control of guess values

Hi,

it�s great for displaying and analyzing data. The manufacturer is OriginLab (formerly known as Microcal), Origin6.1 is their most recent version (I always would recommend the PRO) and their website is www.originlab.com . They provide Demo-versions.

It would be great, to have some examples, which combine Mathcad with Origin (should be possible with scripting, but I am no OLE- or VB-wizard).

Cheers,
Hubert

## Regression - visual control of guess values

Thanks Valery and Hubert.
I got used to approximate functions through my profession in Instrumentation. My tools are quite simple: Polynomials, Thi�l� (which I love so much and complements so nicely Lagrange).
I have a short library of remarkable functions.
In complement I wish I would have:

1. a tool to extract the 'an' coefficients for ChebyShev ( input [x : y ] and return the final an ....something like the approximation of Neperian Log. We have a case in cryogenic semiconductors)
2. For certain type of curve fitting, I have my own unpublished tool to extract rational fractions:
P(x)/Q(x). But I don't have a generalized method.

3. The issue is to have Mathcad execute in simple modules. My Fortran is so far away.

All suggestions are welcome.
Thanks: Jean

## Regression - visual control of guess values

Sorry! But I does not say that the *visible* guess values are the best guess values. I only say that we can use the visualization for solve this task.