[R] Summarising by group

[Ista Zahn]

Trolling? (but see in line below)

On Mon, May 9, 2011 at 5:12 AM, Martyn Byng <Martyn.Byng at nag.co.uk> wrote:
> 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

The wealth of R packages is a core strength of the platform. It is not
a disadvantage to have a wealth of well-developed code for almost any
statistical application.

(see sample
> code on Analyticbridge to do it), and the solution is painful,

Well, the Analyticbridge code is painful, but that example says far
more about the person who wrote it that it does about R. All they
really needed to do was

v <- ddply(xx, .(country), summarize,
           COR = cor(income, age),
           MEAN_age = mean(age),
           MEAN_income = mean(income),
           MAX_income = max(income),
           STDEV_income = sd(income))

I'm not intested in signing up so I can post a reply to the orignial
post, but feel free to copy my answer there if you want.

Best,
Ista

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."

Wrong, see example above.

Best,
Ista
>
>
>
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-- 
Ista Zahn
Graduate student
University of Rochester
Department of Clinical and Social Psychology
http://yourpsyche.org