Hi all, sory f it is too simple.... I am new on using R and have not been able to find the answer of my problem in the help archives. I am performing a relatively simple aov aov( var ~ block + block/plot/treatment + season:bloc....) it's a bit longer but this is how it looks like. my var is "lipid content", "starch content",...in wood samples. Now I want to add the size of the tree from where the sample was taken as a covariable, because I think it could have something to do. How can I declare it ? Thanks in advance Bernat [[alternative HTML version deleted]]