Best Poly Fit Coefficients do not agree with Best Poly Fit 
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 Best Poly Fit Coefficients do not agree with Best Poly Fit

Using General Poly Fit.vi (with Givens algorithm, although others
behave similarly), I found that the Best Polynomial Fit data array fit
my data very well but the corresponding Polynomial Fit Coefficients
did not.  This is surprising since I would have thought that the Best
Polynomial Fit data array would have been generated from the Poly Fit
Coefficients.  It appears the difference is a small offset between 5
and 20 on the Y-values.  I used MS Excel to check the Poly Fit
Coefficients output with the Best Poly Fit data array otput.  I am
using a 5-th order fit with an array of 59 data points.  The input
x-data ranges from 363.6 to 386.8 at 0.4 spacing.  The input Y-values
range from 149.75 to 974.29.  The input Y-data is well behaved
almost linear.  Should I expect this difference between the fit
coefficients and data array output?  I could add in an offset to
correct the Best Poly Fit Coefficients; but this correction shouldn't
be necessary.

Danny



Thu, 18 Aug 2005 04:03:28 GMT  
 Best Poly Fit Coefficients do not agree with Best Poly Fit
Hi Tim,

Thanks so much for the very quick reply.  I tried my actual numbers
using your vi and found that my results were good.  That forced me to
look harder at my code.  Since I was using the Differentiation.vi, I
needed to convert the Poly Fit Coefficients to a string for use in the
Differentiation.vi formula input.  To do this, I used the Number to
Exponential String.vi.  Well, the default precision for this vi is 6
whod a thought that.  As soon as I changed it to 8 the numbers came
out much better.  I could have bumped it up more for even better
results; but, 8 was sufficient.

You were right on the mark!  Thanks again!

Danny



Thu, 18 Aug 2005 23:25:16 GMT  
 
 [ 2 post ] 

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