I've worked on a brief explanation of how to fit an S-shaped curve using SPSS and someone wanted to know how to do this in S-plus. Here's a simple example.
xc <- c(0.8,0.5,0.2,-0.2,-0.5,-0.8)
yn <- c(0.02,0.10,0.29,0.20,0.80,1.00)
nls.mod1 <- nls(yn~1/(1+exp(b*xc-a)),start=list(a=0,b=10))
summary(nls.mod1)
Formula: yn ~ 1/(1 + exp(b * xc - a))
Parameters:
Value Std. Error t value
a -2.70380 1.4019 -1.92867
b 7.90714 3.7126 2.12981
Residual standard error: 0.149848 on 4 degrees of freedom
Correlation of Parameter Estimates:
a
b -0.908
plot(xc,yn,xlim=c(-1,1))
x.grid <- seq(-1,1,by=0.01)
lines(x.grid,predict(nls.mod1,newdata=list(xc=x.grid)))
This same code should also work in R.