# Re: [R] how to calculate the mean of a group in a table

From: Pete Brecknock <Peter.Brecknock_at_bp.com>
Date: Sat, 07 May 2011 11:56:02 -0700 (PDT)

tornanddesperate wrote:
>
> Hi its me again
>
> I don't mean to get on your nerves, but the use of R proofs to be a bit
> more complicated than envisaged.
>
> I would like to calculate the mean of a group of values, here
> "wage_accepted". The group is determined by the stage and period, so in
> the end there should be a column with the values of the wages in period 1
> stage1, period 1 stage 2, period 2 stage1... Unfortunately, I haven't much
> of a clue on how to program this. Could you tell me how the function
> should roughly look like – if-else, loops included or not – in the end ?
>

A couple of possible approaches ....

lines <-"treatment session period stage wage_accepted type

```1       1      1     1            25  low
1       1      1     1            19  low
1       1      1     1            15  low
1       1      1     2            32 high
1       1      1     2            13  low
1       1      1     2            14  low
1       1      2     1            17  low
1       1      2     1            12  low"

```

# 1. using the aggregate function
aggregate(d\$wage_accepted, list(period=d\$period, stage=d\$stage), FUN=mean)

# 2. using the by function
by(d\$wage_accepted,list(d\$period,d\$stage),FUN=mean)

As for tutorial resources, I would recommend visiting CRAN at http://cran.r-project.org/ ... follow the "Manuals" link on the left hand side of the page.

HTH Pete

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