[R] Summarising by group

From: Martyn Byng <Martyn.Byng_at_nag.co.uk>
Date: Mon, 09 May 2011 10:12:57 +0100

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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