With the help of Andrie on StackOverflow.com, I was able to learn about
ddply.  I have another question that is more trivial and cannot seem to find
help on IRC and do not want to bother Andrie again.  I can't seem to figure
out what to google for, so I thought I'd ask here.

I have:
library(plyr)
df_diff <- ddply(df, .(SOURCE), summarize,
TIME_DIFF=-unclass(diff(REQUEST_DATE)))
df_diff
  SOURCE TIME_DIFF
1      A      7.55
2      A      5.55
3      A      3.40
4      D     35.00
5      D    563.00
6      D     37.00
7      D     35.00
8      D    996.00

... with a lot more records.

I want to essentially sort SOURCE asc, TIME_DIFF asc and output the top 15
lowest TIME_DIFFS for each SOURCE.  How do I do this?

Also, what is the data type of df_diff called so that I can look into it
some more?

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