Thank you Michael, that worked perfectly! 

Now I wonder, if it is possible to break my data further apart and put it
together again. 
Assume I include a column for an ID in the data frame like this: 

dattrial2<-data.frame(a=c(1,NA,NA,NA,2,3), Week=c(3,3,3,4,4,4),
AnimalID=c("Ernie","Bert", "Ernie", "Bert", "Bert", "Ernie"))

Is it possible to get two different lists in the output, one for Ernie and
one for Bert? Or do I have to do it seperately for each animal?

Thank you again! I learn a lot by doing and by people helping me. Thank you
for the hint with the paper. 





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