[proFit-list] Can I fit a function to data with asymmetric errors?

[Scott Lasley]

This may not be helpful, but the stackoverflow answer from DanHickstein mentions that the PAIDA python package can do fits with asymmetric weighting.  
https://stackoverflow.com/questions/19116519/scipy-optimize-curvefit-asymmetric-error-in-fit
http://paida.sourceforge.net/documentation/fitter/index.html
http://paida.sourceforge.net/documentation/fitter/sampleFitter_asymmetric.py

I've never used PAIDA and it was last updated in 2007 so I'm not sure how well, or even if, it works with current python versions and libraries.

The last entry in this google gnuplot forum mentions using the simplex algorithm
https://groups.google.com/forum/#!topic/comp.graphics.apps.gnuplot/9435qi8L-yE

Finally, google found this on stackoverflow.  See the "Update on asymmetric errors" section
https://stackoverflow.com/questions/46832659/python-power-law-fit-with-upper-limits-asymmetric-errors-in-data-using-odr


> On Feb 13, 2020, at 4:02 AM, Jan LAGERWALL <jan.lagerwall at uni.lu> wrote:
> 
> Hi all,
> 
> I am fitting an equation to data that has quite large errors at one end of the measuring range and considering the error estimates is essential to getting good results. A problem is, however, that it seems fitting always assumes symmetric errors. My y-values are volume fractions, which obviously cannot be greater than 1, but as I approach volume fraction 1, my estimated errors are on the order of 0.2 or 0.3. This means that a value of 0.9 is considered going from, say, 0.7 to 1.1 during fitting, but of course values greater than 1.0 do not make sense.
> 
> In another data set, I have x-values that I know are lower estimates. In other words, here I would like to set x-error downwards equals zero but the error upwards can be significant. I can plot the data with asymmetric error bars, but I cannot find a way to tell the fitting algorithm to consider asymmetric errors. 
> 
> Am I missing something here, or did I misunderstand something fundamental about how the fitting works, or can I tweak the fitting so asymmetric errors are being considered?
> 
> Thanks a lot!
> /Jan
> 
> 
> 
> Dr. Jan P. F. Lagerwall 
> Professor
> Department of Physics and Materials Science
> 
> Université du Luxembourg
> 
> Campus Limpertsberg
> 162a, avenue de la Faïencerie, BS 1.15a
> L-1511 Luxembourg
> 
> T +352 46 66 44 6219
> F +352 46 66 44 36219
> Jan.Lagerwall at uni.lu /www.lcsoftmatter.com
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