Assignment #3

I don't think that you actually need to add any columns, one could just change the 0d coding into a 0 for all intensive purposes.

I think the easiest way to do the tally is to convert the date time into posix format and then prototype the cum function on a subset of the data (for one id). Basically you just want to create a vector of first differences of the datetime and then multiply these by the dosage...Let me know if that makes sense.