Re: interpretation of fit errors

[Daniel J S. <dan...@ie...>]

On 05/05/2011 04:10 PM, Bastian Märkisch wrote:
>> But if it is some nuanced detailed that initially could be seen as a
>> mistake in coding, then I'd say backward compatibility isn't so much an
>> issue.
>
> I am pretty sure that this was a deliberate choice. The reasoning being
> that as long as the fit is good, FIT_STDFIT is somehow close to 1. So it
> wouldn't hurt too much if "real" errors were given. See
> http://article.gmane.org/gmane.comp.graphics.gnuplot.devel/3737

I don't know about the argument that once the fit isn't so good it 
doesn't matter whether the errors are normalized or not.

One thing I notice is that Hans-Bernhard uses the term "residuals". 
Now, "residuals" is a fairly common definition in fitting.  Maybe it 
would have been better in the first place to use "_res" extensions to 
variable names for the unscaled errors and "_err" for the scaled errors 
(or maybe "_ferr" for fitting error with the inherent meaning that 
fitting errors are always scaled).

The argument is made in the post that one is derived from the other with 
a simple scaling; a scaling which is made available to the user so 
redundant information is given.  True, but I don't know if minimal 
representation is that important.  If we're talking minimal basis or 
some linear algebra concept, sure.  But my main point in all this is to 
avoid ambiguity.  If "error" has some ambiguity in the field, whereas 
"residual" is much less ambiguous, then go with the latter.


>> My fear with this is that a user could run the fit, get the results and
>> significantly misinterpret what they mean by assuming errors were
>> expressed as scaled or unscaled.
>
> That is already the case. Most physicists I know incorrectly assume that
> gnuplot reports "unscaled" errors.

That's not good.


> Why not give them means to get what
> they expect?

Yes, naturally.  But the most coherent way to do that is the question, 
right?  We want gnuplot to be easy to use, not arcane.

Dan