Given what you have tried, I wonder whether the problem is that 'cavg' and 'havg', produced by tapply, are not the simple vectors that 'rlm' expects. A simple way to check:
cavg <- as.vector(cavg) havg <- as.vector(havg)
then run rlm.
That worked! Many thanks. Has there been a change in either MASS or R in this regard since R 1.7? It was only after updating to R 1.8.1 that I had this problem.
Thanks again to all for your rapid replies,
Ed.
-- Edward H. Hagen Institute for Theoretical Biology phone: +49/30 2093-8649 Humboldt-Universit�t zu Berlin fax: +49/30 2093-8801 Invalidenstra�e 43 http://itb.biologie.hu-berlin.de/~hagen 10115 Berlin, Germany
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