| Yet another user with poly_fit problems [message #86074] |
Mon, 30 September 2013 12:59  |
Gus
Messages: 7
Registered: September 2013
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Junior Member |
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Hello everyone,
I've read a few of the older posts on this topic, but their solution didn't really help me solve the problem that I am currently having with the poly_fit function. The set of coefficients generated by the function (a 4th degree polynomial) produces some rather absurd results. Here is a short version of the problem I am having.
X = [0.000000, 11.6667, 822.914, 3458.85, 27703.4, 133928.]
Y = [15.9000, 16.0000, 17.0000, 18.0000, 19.0000, 20.0000]
C = poly_fit(X, Y, /double, yfit=D)
IDL generates the following coefficients (for C)
15.940691
0.0015355228
-3.0965110e-007
1.1170193e-011
-6.6767399e-017
Yet, one will clearly see that this fit produces rather undesirable results since, within the same range of X values (0 to roughly 150,000), this fit will produce Y values that can be as high as 1600 and as low as -3000 (rather than between 15.9 and 20). Excel is generating better coefficients than IDL!
Here is what I have already tried to do (and did not solve the problem)
1) Double precision of X and Y prior to using the poly_fit function (notice that I am using the "/double" keyword function already in that function);
2) Subtracting the mean of X from that array, before fitting the data - suggested in previous posts;
3) Subtracting the value of X[0] from that array, before fitting the data;
4) Subtracting the mean of Y from that array, before fitting the data.
Does anyone know of any other solution to this problem?
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