Within ID, check for matches/differences

Question

I have a large dataset, over 1.5 million rows, from 600k unique subjects, so a number of subjects have multiple rows. I am trying to find the cases where the one of the subj

Answer 1

One approach using plyr:

library(plyr)
  zz <- ddply(test, "ID", summarise, dups = length(unique(DOB)))
  zz[zz$dups > 1 ,]

And if base R is your thing, using aggregate()

zzz <- aggregate(DOB ~ ID, data = test, FUN = function(x) length(unique(x)))
zzz[zzz$DOB > 1 ,]