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

[Jan LAGERWALL]

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<mailto:Jan.Lagerwall at uni.lu> /www.lcsoftmatter.com<http://www.lcsoftmatter.com>
physics.uni.lu<http://physics.uni.lu>, www.uni.lu<http://www.uni.lu>






-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://quantum-soft.com/pipermail/profit-list_quantum-soft.com/attachments/20200213/dc0db2f1/attachment.html>