Re: fit ignores yrange

[sfeam <sf...@us...>]

On Saturday, 06 June 2015 06:59:45 PM pl...@pi... wrote:
> >
> > gnuplot> fit [][ymin:ymax] f(x) 'data' via a,b
> > Warning: bracketed range on "y" is ignored
> >
> > or
> >
> > gnuplot> fit [][ymin:ymax] f(x) 'data' via a,b
> > Error: Second bracketed range in a 1-parameter fit must begin with z=
> >

> Thanks Ethan.
> 
> that may be a start but I think the issue of whether this is legitimate 
> application of least squares regression needs to be addressed.
> 
> x is the independant variable thus any limits on the data range should 
> be expressed in x coordinates.

>From a data-centric viewpoint, I don't think I buy that argument.
When both x and y are experimentally determined values, why should
you be able to limit the acceptable range of one but not the other?

> Since y ( or z ) values are assumed by the fitting algo to be error 
> laden using them to select a range is not valid.  

I do not follow your line of thinking there, unless you are
suggesting that the limit should be applied to (y +/- yerror)
rather than to y.

Anyhow, there can also be errors associated with x.

I realize the default form of the fit command does not handle errors on x,
but version 5 introduced an option "fit xyerror" that does.

> If there is a 
> need/desire to do this maybe the regression should be being done the 
> other way around.
> 
> There are huge problems in many fields of science from the lack of 
> appreciation of the assumptions and pre-conditions required for 
> least-squares to provide a result that is a valid estimator of the 
> regressed function.
> 
> Perhaps someone could suggest why this feature is there at all and in 
> which circumstances it can be considered a legitimate application of LSQ.

If you mean "why do we allow bracketed ranges in the fit command?"
the only justification I see is backwards compatibility.
I recommend against using bracketed ranges in any of the commands,
'plot', 'fit', or 'stats'.  

If you mean "why do we allow filtering on z range?", I think that
is a case of providing a tool and leaving it up to the user to make
responsible use of it.   I don't use gnuplot's fit routine that much,
and when I do I usually pre-filter the data in an external tool before
giving it to gnuplot.  But there are times when it is convenient
to filter on the range of either x or y in 2D experimental data,
or on the range of all observed values in higher dimensional data.

	Ethan