Question: Curve Fitting not working

Hi,

I am kind of new with using maple, and I am hoping that someone can give me some help on nonlinear curve fitting.  I checked out the help topics, and I thought that I had put everything in correctly, as per the help file.  However when I try to do a fit to data, nothing is returned.  Here is what I have input:

>    with(Statistics)

>

>     data := readdata("D:/Research/redShiftReduced.csv", 67)

>     data2 := readdata("D:/Research/chiReduced.csv", 67)

>

>            redShift := Vector(data, datatype = float)

>            chi := Vector(data2, datatype = float)

>

> "f(z):= (9.01554908299*10^(-5)*a^(((-1)/2))*(a-b-(a+b)*(1+z)^((3*b)/a)))/(c*(((1+z))^((3*b)/a)-d)):"

>                   fit(f(z), redShift, chi, z):

I am trying to get a,b,c,d with z as an independent variable.  Am I missing something in my commands that I don't know?  Any help would be appreciated, thanks!