How to Calculate the Mean by Group in R (With Examples)

Often you may want to calculate the mean by group in R. There are three methods you can use to do so:

Method 1: Use base R.

aggregate(df$col_to_aggregate, list(df$col_to_group_by), FUN=mean)

Method 2: Use the dplyr() package.

library(dplyr)

df %>%
  group_by(col_to_group_by) %>%
  summarise_at(vars(col_to_aggregate), list(name = mean))

Method 3: Use the data.table package.

library(data.table)

dt[ ,list(mean=mean(col_to_aggregate)), by=col_to_group_by]

The following examples show how to use each of these methods in practice.

Method 1: Calculate Mean by Group Using Base R

The following code shows how to use the aggregate() function from base R to calculate the mean points scored by team in the following data frame:

#create data frame
df <- data.frame(team=c('a', 'a', 'b', 'b', 'b', 'c', 'c'),
                 pts=c(5, 8, 14, 18, 5, 7, 7),
                 rebs=c(8, 8, 9, 3, 8, 7, 4))

#view data frame
df

  team pts rebs
1    a   5    8
2    a   8    8
3    b  14    9
4    b  18    3
5    b   5    8
6    c   7    7
7    c   7    4

#find mean points scored by team
aggregate(df$pts, list(df$team), FUN=mean)

  Group.1        x
1       a  6.50000
2       b 12.33333
3       c  7.00000

Method 2: Calculate Mean by Group Using dplyr

The following code shows how to use the group_by() and summarise_at() functions from the dplyr package to calculate the mean points scored by team in the following data frame:

library(dplyr) 

#create data frame
df <- data.frame(team=c('a', 'a', 'b', 'b', 'b', 'c', 'c'),
                 pts=c(5, 8, 14, 18, 5, 7, 7),
                 rebs=c(8, 8, 9, 3, 8, 7, 4))

#find mean points scored by team 
df %>%
  group_by(team) %>%
  summarise_at(vars(pts), list(name = mean))

# A tibble: 3 x 2
  team   name
  <fct> <dbl>
1 a       6.5
2 b      12.3
3 c       7

Method 3: Calculate Mean by Group Using data.table

The following code shows how to calculate the mean points scored by team in the following data frame:

library(data.table) 

#create data frame
df <- data.frame(team=c('a', 'a', 'b', 'b', 'b', 'c', 'c'),
                 pts=c(5, 8, 14, 18, 5, 7, 7),
                 rebs=c(8, 8, 9, 3, 8, 7, 4))

#convert data frame to data table 
setDT(df)

#find mean points scored by team 
df[ ,list(mean=mean(pts)), by=team]

   team     mean
1:    a  6.50000
2:    b 12.33333
3:    c  7.00000

Notice that all three methods return identical results.