I am trying to look at photosynthetic rate over time, but it decreases over the growing season, so I think I need the quadratic term (w2 = week*week; w = week). But this is the output I get - without any error messages from the SAS system:
Is the model simply out of degrees of freedom? I have 123 samples, and when I remove some of the interactions from the model to free up some df, the model still will not fit any quadratic terms.
Someone has suggested that the problem here is that w and w2 are completely confounded with the class variable week, and so there are no degrees of freedom to estimate these parameters. But when I remove week as a class variable from the model, I get the following message:
ERROR: Only class variables allowed in this effect.