[datatable-help] Contrast matrix with a continuous variable

[CRoa]

Hi all,
I was wondering if someone could help me designing a contrast matrix when
you have a continuous variable (Days).  

My model looks like this: 

model<-lme(Y~A*B*Days, data=data_over_time) 

The factor A has two levels (A1 and A2) and factor B has three levels (B1,
B2, and B3). I measured the response variable Y every two or three days over
70 days (Days).  
I need to look at only a few comparisons over the 70 days such us: 
                                   A1 and B1 vs A2 and B1,  
                                   A2 and B2 vs A2 and B1,  
                                   A1 and B2 vs A2 and B2
I could use the function contrast from the package contrast to design the
matrix with the three comparisons mentioned above for specific days at a
time. However, I need to run the comparison over the 70 days not at
individual time points. 

I was wondering if someone could help me designing this contrast matrix. 

Any help would be much appreciated
Cheers



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