Hi to all, I found in the R-help archive how to calculate the p-value for a gee result: http://finzi.psych.upenn.edu/R/Rhelp02a/archive/74150.html but there are two questions (I am afraid they are basic questions ...) 1. why is the result multiplicated with 2 2. how could I decide between lower.tail =TRUE and FALSE: example: 2*pnorm(c(1.8691945,0.5882351,2.4903091,1.9287802,2.3172983,2.2092593,2.2625959,1.6395695), lower.tail =TRUE) I did not find anything about lower tail in my books only about one tailed and two tailed. With regards Carmen