Options when drawing using the Curve Fitting tool

I am inputting x y z data and using 'Curve Fitting tool' to create a nonlinear regression graph.

I am trying to find the coefficients a,b,c,d suitable for the data using the 'Custom Equation' formula, but the graph is too different from the data.

I tried changing Robust and Algorithm, but the graph is not similar to the data.

What additional work should I do to get the graph?

Answer 1

[Alex Sha]

Hi, post your data and function in txt format, if possible.

[haeyeon JI]

Thanks for answering my question.

[Walter Roberson]

The function in the image appears to be a*x*b*log10(y+c)+d ?

If so then a cannot be distinguished from b: you can only distinguish their product.

[haeyeon JI]

It is an expected expression,

and my expression is ax+blog10(y+c), not ax*blog10(y+c).

And please note that the data has been modified

[Alex Sha]

The fitting result is not so good if the function is: z=a*x+b*log10(y+c)

Root of Mean Square Error (RMSE): 0.115584191368428
Sum of Squared Residual: 0.654625559420376
Correlation Coef. (R): 0.855538827407886
R-Square: 0.731946685202461
Adjusted R-Square: 0.714076464215958

Parameter	Best Estimate
----------	-------------
a	0.0741121571659251
b	-0.283418916981476
c	-9.47213514559854