Mean per group in a data.frame

Question

I have a data.frame and I need to calculate the mean per group (i.e. per Month, below).

Name     Month  Rate1     Rate2
Aira       1      12        23
Aira       2      18        73
Aira       3      19        45
Ben        1      53        19
Ben        2      22        87
Ben        3      19        45
Cat        1      22        87
Cat        2      67        43
Cat        3      45        32

My desired output is like below, where the values for Rate1 and Rate2 are the group means. Please disregard the value, I have made it up for the example.

Name       Rate1       Rate2
Aira        23.21       12.2
Ben         45.23       43.9
Cat         33.22       32.2

Answer 1

This type of operation is exactly what aggregate was designed for:

d <- read.table(text=
'Name     Month  Rate1     Rate2
Aira       1      12        23
Aira       2      18        73
Aira       3      19        45
Ben        1      53        19
Ben        2      22        87
Ben        3      19        45
Cat        1      22        87
Cat        2      67        43
Cat        3      45        32', header=TRUE)

aggregate(d[, 3:4], list(d$Name), mean)

  Group.1    Rate1    Rate2
1    Aira 16.33333 47.00000
2     Ben 31.33333 50.33333
3     Cat 44.66667 54.00000

Here we aggregate columns 3 and 4 of data.frame d, grouping by d$Name, and applying the mean function.

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Or, using a formula interface:

aggregate(. ~ Name, d[-2], mean)