[R] Calculation of r.squared for linear model with offset

[Ross Darnell]

R^2 for a model is usually defined as 1-RSS/TSS where TSS is the SS 
about the mean and RSS is the residual SS from the model.

Consider the model in R

z <- runif(20)
y <- z+rnorm(20)
my.model <- lm(y~offset(z))
summary(my.model)$r.squared

Here the RSS is equivalent to the TSS and
gives 0 when it should (IMHO and a few others perhaps) be

1 - RSS/TSS(corrected for the mean only)

RSS = sum(resid(my.model)^2)
TSS = sum((y-mean(y))^2)

Have I missed this somewhere in the FAQ's or elsewhere?

Regards
Ross Darnell