I want to be able to exctract a matrix from a data frame that contains repeated measurements for individuals. i.e I want to exctract the second observation for each individual in the study. 
Is there a way of doing this? 

I guess what I am trying to ask is if there as a multidimensional version of aggregate() as in aggregate(data, list(data[,"ID"]), function(x) x[2,])
but aggregate will only give back scalars. 

Thank you, 
Jean
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