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Re: Semi-log and log graphs



Having read what others are saying about logarithmic graphs, I just wanted
to point out (and I'm sure that they know this) that it is incorrect to do
a linear fit on the transformed (i.e., "logarithimized") data, in any
case. It is incorrect even when the uncertainties are weighted, a.k.a.
weighted least-squares or equivalent.

May I point out here that "incorrect" is too strong a term? The fitting
of a curve to data is a process which is used "to guide the mind". The
use of a least squares algorithm is entirely a matter of aesthetic
choice; no specific fitting algorithm is mandated by Nature.

Thus, if one makes it clear that the fit made to data in a given case
is a linear fit to a logarithmic series derived from the data, then
that is perfectly correct. It would also be correct to fit an
exponential to the data by a least squares method and specify that it
was done that way. That would likely give a different fit, but the fit
would not be in any way intrinsically superior to the first one. In
order to make a judgment of relative merit one would need more
information about the object of the measurements.

Leigh