How to do 4PL Logistic Regression

From: "youjie@xxxxxxxxx" <youjie@xxxxxxxxx>
Date: Wed, 13 Feb 2008 07:46:15 -0800 (PST)

I am working on curve fitting a 4 parameters logistic regression
model.
Given a set of points (x, y) and calculate the 4 parameters.

The 4PL equation is

y = D + ( A - D ) / ( 1 + (x/c )^B)

To do the curve fit, what should I do? Or the method?

I already finished fitting a 2 parameters logistic model.

Thanks.
.

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