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

[tornanddesperate]

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 ?

   treatment session period stage wage_accepted type
1          1       1      1     1            25  low
2          1       1      1     1            19  low
3          1       1      1     1            15  low
4          1       1      1     2            32 high
5          1       1      1     2            13  low
6          1       1      1     2            14  low
7          1       1      2     1            17  low
8          1       1      2     1            12  low

I am also very grateful if someone could point out a good internet tutorial
resource for R functions such as if-else and loops and how to use them in
conjunction with tables. It should contain simple examples if possible. 

As ever indepted to you,

Matthias



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