[SciPy-User] How to estimate error in polynomial coefficients from scipy.polyfit?

Jeremy Conlin jlconlin@gmail....
Mon Mar 29 10:08:48 CDT 2010


On Thu, Mar 25, 2010 at 9:40 PM, David Goldsmith
<d.l.goldsmith@gmail.com> wrote:
> On Thu, Mar 25, 2010 at 3:40 PM, Jeremy Conlin <jlconlin@gmail.com> wrote:
>>
>> Yikes!  This sounds like it may be more trouble than it's worth.  I
>> have a few sets of statistical data that each need to have curves fit
>> to them.
>
> That's an awfully generic need - it may be obvious from examination of the
> data that a line is inappropriate, but besides polynomials there are many
> other non-linear models (which can be linearly fit to data by means of data
> transformation) which possess fewer parameters (and thus are simpler from a
> parameter analysis perspective).  So, the question is: why are you fitting
> to polynomials?  If it's just to get a good fit to the data, you might be
> getting "more fit" than your data warrants (and even if that isn't a
> problem, you probably want to use a polynomial form different from "standard
> form," e.g., Lagrange interpolators).  Are you sure something like an
> exponential growth/decay or power law model (both of which are "more
> natural," linearizable, two-parameter models) wouldn't be more appropriate -
> it would almost certainly be simpler to analyze (and perhaps easier to
> justify to a referee).
>
> On this note, perhaps some of our experts might care to comment: what
> "physics" (in a generalized sense) gives rise to a polynomial dependency of
> degree higher than two?  The only generic thing I can think of is something
> where third or higher order derivatives proportional to the independent
> variable are important, and those are pretty uncommon.

I will only be fitting data to a first or second degree polynomial.
Eventually I would like to fit my data to an exponential or a power
law, just to see how it compares to a low-order polynomial.  Choosing
these functions was based on qualitative analysis (i.e. "it looks
quadratic").

The best case scenario would be that I take what I learn from this
"simple" example and apply it to more difficult problems as they come
along down the road.  It appears, however, that it's not so simple to
apply it to other problems.  I wish I had more time to learn about
fitting data to curves.  I'm sure there are a lot of powerful tools
that can help.

Jeremy


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