# Re: Polynomial regression - Warning

• From: Prof J C Nash <nashjc ncf ca>
• To: gnumeric-list gnome org
• Subject: Re: Polynomial regression - Warning
• Date: Tue, 09 Oct 2007 15:46:07 -0400

The type of dialog Adrian suggests would be a very sensible feature. Sometimes one really does need to do the polynomial regression, even if to show the issues, so it is not right to completely bar an approach. However, when we have an interested person, it's a good opportunity to point them towards potentially better methods. There was some discussion of such dialogs in the 80s for statistical software, and there's some of these ideas in some of the packages. Certainly I'm interested in helping out on this sort of thing, and have been gradually clawing away at other stuff I have to do so I can return to the test spreadsheets, which are one way to bring the issues to light.
```
JN

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```Lovely repartee, just the sophisticated answer that gnumeric brings to

Any chance you can craft this into a good popup dialog? e.g.

You are trying to use SOME_METHOD which exists in gnumeric only
to allow compatibility with other older spreadsheet programs.
The computations involved in that method are known to be
exceedingly problematic. However, you might be able to solve
your problem using SOME_OTHER_METHOD. Would you like to use that

Of course, this is all over my head. I'm merely hoping to leverage your
40 years of knowledge into a helpful dialog.

On Tue, 2007-10-09 at 14:46 -0400, Prof J C Nash wrote:
```
Polynomial regression was where I came into numerical analysis at the beginning of my academic career 40 years ago. It is a dangerously ill-conditioned problem, meaning that the regression parameters are untrustworthy, though the "fit", i.e., the model, may be useful if the calculations are done properly. There are ways of doing it that are "less dangerous" using orthogonal polynomials.
```
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Just a warning that it should not be a high priority to add. Let the Excel users drive off the cliff. If it is added for Excel compatibility, then I'd still recommend a "Gnumeric does Vista" and pop up warnings asking if the dangerous move should be allowed. Come to think of it, one could borrow exactly the Vista popups so folk would blame Bill.
```
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Seriously, think if you really need polynomial regression or can use a less troublesome approach.
```
JN

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