Problems with exponential fit.

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Todd Gross
Todd Gross 2015-11-30
评论： Todd Gross 2015-12-1
I start with the following data:
x,y
0.0025,10
0.001818,100
0.001429,10000
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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回答（1 个）

Walter Roberson
Walter Roberson 2015-12-1
  1 个评论
Todd Gross
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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