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 R.

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)
```

You can find an earlier version of this page on my original website.