| Re: MPFIT parameter errors -0.00000 [message #76058 is a reply to message #75989] |
Thu, 12 May 2011 07:31  |
Craig Markwardt
Messages: 1869
Registered: November 1996
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Senior Member |
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On May 9, 4:35 pm, JoeM <josephmeir...@gmail.com> wrote:
> Hello,
>
> I was wondering about something strange that seems to be going on
> with MPFIT in IDL. MPFIT seems to be working and producing a fine
> looking fit, but the errors on some of the parameters are identically
> -0.00000. These parameters are not limited or tied to anything at the
> moment, so they should be producing some parameter error. Output is
> attached below, MPFIT is ending with a "2" status. Any idea what is
> going on here? This only seems to happen for a specific data file, for
> other fits it seems to be working fine.
>
> Cheers,
> Joe
>
> IDL>
> % MPFIT: WARNING: data is DOUBLE but parameters are FLOAT
> % MPFIT: (converting parameters to DOUBLE)
> Iter 1 CHI-SQUARE = 148.65044 DOF = 99
> P(0) = 2.70126E+16
> P(1) = 2.02481E+16
> P(2) = 32.2569
> P(3) = 37.7481
> P(4) = 0.000100000
> P(5) = 71.0903
> P(6) = 0.273900
> P(7) = 0.273900
> P(8) = 930.748
> P(9) = 930.748
> Iter 1 CHI-SQUARE = 148.65044 DOF = 99
> P(0) = 2.70126E+16
> P(1) = 2.02481E+16
> P(2) = 32.2569
> P(3) = 37.7481
> P(4) = 0.000100000
> P(5) = 71.0903
> P(6) = 0.273900
> P(7) = 0.273900
> P(8) = 930.748
> P(9) = 930.748
>
> STATUS
> 2
>
> PERROR
> -0.00000 -0.00000 0.492911 1.38439 0.00000
> 0.930223 0.00000
> 0.00000 0.00000 0.00000
You might find helpful information in the FAQ, in particular...
http://www.physics.wisc.edu/~craigm/idl/fitqa.html#parstep
It looks like you have a dynamic range problem, given that they cover
a range of 1e20!!! Consider re-writing your user function so that the
parameter values are roughly the same magnitude, and also vary by
about the same magnitude. (Which may mean in your case to fit an
offset to a large value instead of the large value itself.)
Also, this error message,
> % MPFIT: WARNING: data is DOUBLE but parameters are FLOAT
> % MPFIT: (converting parameters to DOUBLE)
is ominous. You should strive to maintain consistent numerical
system, either all single precision or all double precision.
These suggestions will be true, no matter which fitting software you
use.
Happy fitting,
Craig Markwardt
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