Passing a variable into the fittype function

Hello,

I have an analytical equation that I wish to fit to some data. I tried it out using the cftool command and it works well. From cftool I automatically generated an m-file in which part of the fit routine looks like this:

ft_ = fittype('2*erf(x)/a+b',... 'dependent',{'y'},'independent',{'x'},... 'coefficients',{'a', 'b'});

where a and b are the coefficients I allow the algorithm to play with to get the best fit.

The problem is that I want to introduce a new variable, c, which changes as follows:

ft_ = fittype('c*erf(x)/a+b',... 'dependent',{'y'},'independent',{'x'},... 'coefficients',{'a', 'b'});

If I do this I get the error message: undefined variable 'c' etc..

I've tried calling a separate function from within fittype where I bring in c from elsewhere, but I get the same error message:

ft_ = fittype('Analytical(x,a,b,c)',... 'dependent',{'y'},'independent',{'x'},... 'coefficients',{'a', 'b'});

Is there something obvious I'm doing wrong?

Best Regards

Pete