Possible bug in rlm

Dear all,

I believe I have found a bug in rlm in the MASS package. Specifically, the scale estimate can be wrong when there are no outliers. The following code snippet is an example:

dose <- c(0,1,2,0,1,2)
response <- c(0.659,1.633,3.621,1.803,3.093,4.424)
line <- c(1,1,1,2,2,2)
k2 <- seq(1.5,5,by=0.01)
repNA <- rep(NA,length(k2))
scale <- repNA
niter <- repNA
for (i in 1:length(k2)){
  rlm.fit <- rlm(response~dose+factor(line), psi=psi.huber,k=1.345,
                         scale.est="proposal 2",k2=k2[i])
  scale[i] <- rlm.fit$s
  niter[i] <- length(rlm.fit$conv)
}
plot(k2,scale,type="b",col=niter)

For this dataset there are no outliers, so I would expect the scale to be a smooth function of k2 once k2 is reasonably large, certainly for k2 > 2. However, there is a funny jump in the scale estimate around k2 = 2.4, just at the point where the number of iterations to convergence falls from 3 to 1.

Looking at the source code, it appears that on each iteration, the scale is updated, then the parameters, and then a check for convergence is carried out just for the parameters, not the scale. So I would guess that in the range around k2=2.5 convergence is being reached when in fact the scale estimate hasn't converged.

I am using MASS version 7.3-33 and R version 3.1.0 on Windows.

I am not sure how common this issue is but there does not seem to be anything special about my dataset so it could be quite generic. Am I right that this is a bug?

Many thanks,
Francis Bursa

------------------ 
Francis Bursa
Statistician

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