using more than one 'by' variable


I am using a Wilcoxon rank-sum test to analyse some time course data. I have repeated all the experiment twice so I would like to be able to analyses first all the experiments done on searate days before I analyse the combined data from the repeats. I have therefore made a variable called 'day' and a second one called 'experiment' to enable me first to do the analysis eg of day 3, experiment 1 separately from day 3 experiment 2 and then if they show the same trend I will combine them and analyse all the day 3 data at once.

The problem that I am having is that I keep getting an error when I try to sort by two variable at the same time ie day and experiment. Can anyone tell me how I can do this?

Thanks :tup: