[SciPy-user] Estimation of parameters while fitting data

Hullo, I have as my fitting function a system of differential equations(3) and I am supposed to get the optimal parameters to use by fitting data to this system. I tried to use scipy.optimize.leastsq. The difficulty is with the function that calculates the difference between the data and the values from the fitting function. The result of the fitting function is a list with three values and yet I only need one of the values to be subtracted from the data value. I have tried to get something from PyDSTool but all in vain. If you can propose something for me to use or suggest a better method, I will be very grateful. Regards, Doreen.