How to do exponential and logarithmic curve fitting in Python? I found only polynomial fitting.

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How to do exponential and logarithmic curve fitting in Python? I found only polynomial fitting.

Hi, I have a set of data and I want to compare which line describes it the best (polynomes of different orders, exponential or logarithmic).

I use Python and Numpy and for polynomial fitting there is a function polyfit(). But I found no such functions for exponential and logarithmic fitting.

Are there any? Or how to solve it otherwise?

Thank you very much. Tomas

+3 A:

For fitting y = A + B log x, just fit y against log x.

For fitting y = Ae^Bx, take the logarithm of both side gives log y = log A + Bx. So just fit log y against x.

KennyTM 2010-08-08 07:41:32

Thank you, that's perfect, but how do I find the base of the logarithm that suits the best?

Tomas Novotny 2010-08-08 09:34:06

@Tomas: Usually the natural log, but any log works. Just remember that if you use base K, then the equation becomes y = A*K^(Bx).

KennyTM 2010-08-08 09:44:00

So the quality of the fitting (for example R2) is not dependent on the base of the logarithm? Thank you once again, the answers are perfect, very useful, I will give you a point as soon as I reach enough reputation.

Tomas Novotny 2010-08-08 10:38:33

@Tomas: Right. Changing the base of log just multiplies a constant to log x or log y, which doesn't affect r^2.

KennyTM 2010-08-08 11:20:22

This will give greater weight to values at small y.Hence it is better to weight contributions to the chi-squared values by y_i

Rupert Nash 2010-08-08 16:54:15

ansaurus

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How to do exponential and logarithmic curve fitting in Python? I found only polynomial fitting.

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