Suppose I have data organized in the following way:
(P_i, M_j, S_k)

where i, j and k and indexes for sets.
I would like to analyze the data to get for example the following
information:
what is the average over k for
(P_i, M_j)
or what is the average over j and k for P_i.

My question is what would be the way of doing this in R.
Specifically how should I write the data in a csv file
and how do I read the data from the csv file into R and perform these basic
operations.

Thank you.

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