[R] Summarising by group

[Martyn Byng]

I wonder if someone with more experience than me on using R to summarise
by group wants to post a reply to this 

http://www.analyticbridge.com/group/sasandstatisticalprogramming/forum/t
opics/why-still-use-sas-with-a-lot

To save everyone having to follow the link, the text is copied below

"SAS has some nice features, such as the SQL procedure or simple "group
by" features. Try to compute correlations "by group" in R: say you have
2,000 groups, 2 variables e.g. salary and education level, and 2 million
observations - you want to compute correlation between salary and
education within each group.

It is not obvious, your best bet is to use some R package (see sample
code on Analyticbridge to do it), and the solution is painful, you can
not return both correlation and stdev "by group", as the function can
return only one argument, not a vector. So if you want to return not
just two, but say 100 metrics, it becomes a nightmare."



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