Unknown value remains start value - curve fitting toolbox

Hello,

I'm trying to fit the Hertz model (parabolic) to some data using the curve fitting toolbox of MATLAB.

ft = fittype( '(4/3 * sqrt(20E-9)) * (E /(1-0.4^2)) * (d - 0)^(3/2)', 'independent', 'd', 'dependent', 'F' );
opts = fitoptions( ft );
opts.Algorithm = 'Levenberg-Marquardt';
opts.Display = 'Off';
opts.Lower = -Inf;
opts.StartPoint = 1.2E6;
opts.Upper = Inf;
ex = excludedata( xData, yData, 'Indices', [1 2 3... ]);
opts.Exclude = ex;

[fitresult, gof] = fit( xData, yData, ft, opts );

The problem I have is that it doesn't really seem to fit the model to the data. The unknown (Young's modulus E) always stays the same number I give as a start value. For one particular case I know that E should be around 3.8E6 but as you can see from the results E remains at the start value 1.2E6. Although, I'm not quite sure what the values in the brackets represent. Is it like an error of the E value?

General model:
     ans(d) = (4/3 * sqrt(20E-9)) * (E /(1-0.4^2)) * (d - 0)^(3/2)
     Coefficients (with 95% confidence bounds):
       E =     1.2e+06  (6.103e+05, 1.79e+06)

I already played a lot with the number of iterations and the termination tolerance but the problem remains.

Anyone encountered the same problem or has an idea where my mistake is?

thanks a lot in advance!

[Star Strider]

Not an ‘error’ in the brackets, but the 95% confidence limits. Since they don’t include zero, the estimate for ‘E’ is significant.