How to write sigmoid equation into formula string in Nonlinear Curve Fit LM?

[up_horn]

I am using a sigmoid 4 parameter formula to fit a step-like curve,

The formula is y=y0+a/(1+ exp(-(x-x0)/b))

How to write this equation into formula string in Nonlinear Curve Fit LM?

Thanks for any help.

[ttrr]

Hi,

I suppose that a and b are the unknown parameters. You can add y0+a/(1+ exp(-(x-x0)/b)) in model, add a and b in the array of Parameters and add x in x. Note that you should replace x0 and y0 with the actual number.