MATLAB: How to work with CFIT objects in curve fitting

Question

How do I work with CFIT objects in curve fitting?

Answer 1

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/predint.html