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I have a dataframe that contains fields such as patid, labdate, labvalue. The same patid may show up in multiple rows because of lab measurements on multiple days. Is there a simple way to obtain just the first and last record for each patient, or do I need to write some code that performs that. Thanks, Steven

...to plot dates. However, my data are two numeric vectors. The strange part is that these vectors are in fact date differences -- they started life as dates -- but I've already converted them to numeric, like so: $ cogdiff <- as.numeric(cogdate - scandate)/365.24 $ labdiff <- as.numeric(labdate - scandate)/365.24 When I check is(cogdiff) and is(labdiff) it tells me that both are numeric vectors. And if I put these variables into other plotting routines (e.g. plot(), qplot()), I get the expected output. So on some level the conversion to numeric has "worked." However: as a s...