I was wondering if there is a simple explanation for this. I've entered in some of my own equations for non linear fits and have noted that while some may fit the data quite well, the parameters derived from the fit are pretty off. One example is a hyperbolic tangent equation where the fit goes through the data quite well, yet the parameter intended to represent the peak of the curve is considerable higher than the fitted line. I've used the equations before with similar data in other programs, but am pretty new to running the curve fit options in Origin. Thanks for any advice.