I have seen, several times, dots (like this: "y ~." ) in formula descriptions, noticeably in R help. I am unable to see what it does correspond to. Any ideas ? -- --~~ Toulouse, Grenoble, Auch, Arcachon, Béziers, Paris, Saragosse, Lévignac Sur Save, habitats naturel du Valdo. ~~-- < http://www.le-valdo.com>