How do I work with CFIT objects in curve fitting?

How do I work with CFIT objects in curve fitting?

Answer 1

[MathWorks Support Team]

You can use the function CONFINT to find confidence intervals. Full documentation on this function is available at

https://www.mathworks.com/help/curvefit/cfit.confint.html

Here is an example. Suppose that we have exported a fitted model from CFTOOL to the workspace.

fittedmodel1 =
General model:
fittedmodel1(x) = a*sin(x) + b
Coefficients (with 95% confidence bounds):
a =       1.018  (0.938, 1.097)
b =    -0.03808  (-0.09455, 0.01839)

Now use CONFINT to get confidence intervals:

 confint(fittedmodel1)
ans =
0.9380   -0.0946
1.0973    0.0184

Here, the first column is the 95% confidence interval on the first parameter and the second column is the 95% confidence interval on the second parameter. A second input to CONFINT can give other levels of confidence.

To get the actual parameter values, use structure indexing. For example, parameter a could be found by typing

fittedmodel1.a

Another function of interest is PREDINT, to find prediction intervals. Full documentation for this function is available at

https://www.mathworks.com/help/curvefit/cfit.predint.html

[Steven Lord]

For a list of other operations you can perform with cfit and sfit objects see this documentation page.