Problems with exponential fit.

Question

Todd Gross 2015-11-30

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https://ww2.mathworks.cn/matlabcentral/answers/258294-problems-with-exponential-fit

评论： Todd Gross 2015-12-1

I start with the following data:

x,y
0025,10
001818,100
001429,10000

And try to fit to rate=a*exp(bx) using the following code:

p=fit(x,y,'exp1');
p2=coeffvalues(p);
yfit=p2(1)*exp(p2(2)*x);
semilogy(x,y,'.',x,yfit)

and get a poor quality fit along with values that do not make sense. It really underestimates y for larger x.

Excel’s exponential fit seems reasonable.

Any ideas why the Matlab fit does not work. I was working with small x values so I multiplied by 1000 but it had no effect.

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Answer 1

Walter Roberson 2015-12-1

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https://ww2.mathworks.cn/matlabcentral/answers/258294-problems-with-exponential-fit#answer_201730

http://www.mathworks.com/matlabcentral/answers/221316-how-can-i-find-these-values-using-polyfit-command#comment_288984

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Todd Gross 2015-12-1

Using transformed coordinates e.g. log x and log y with a polynomial fit often leads to values other than A for x=0 when y = A*exp(bx). I am interested in learning why exp1 fit does not work well in Matlab when the similar function in Excel seems to work well.

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