[Rd] calculating means per group

[RFTW]

Hi all
I have a very basic question, yet i have not found how to do it.

Suppose my dataset looks like this:

Year	Area	value
1	a	20
1	a	25
1	a	28
1	a	31
1	a	23
1	b	25
1	b	28
1	b	23
1	b	19
2	a	25
2	a	23
2	a	24
2	a	26
2	b	27
2	b	28
2	b	20
2	b	25
2	b	28


Now, i want to calculate a MEAN per year per area. How do i do that?

With mean(value) i calculate the mean of all values of course. I just need
to know how to group year and area correctly.

I assume that i can use this grouping in other calculations too, right?


Cheers,

Luc
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