Relative frequencies / proportions with dplyr

Question

Suppose I want to calculate the proportion of different values within each group. For example, using the mtcars data, how do I calculate the relative frequency of the number of gears by am (automatic/manual) in one go with dplyr?

library(dplyr)

data(mtcars)

mtcars <- tbl_df(mtcars)

# count frequency

mtcars %>%

  group_by(am, gear) %>%

  summarise(n = n())

# am gear  n

#  0    3 15 

#  0    4  4 

#  1    4  8  

#  1    5  5 

What I would like to achieve:

am gear  n rel.freq

 0    3 15      0.7894737

 0    4  4      0.2105263

 1    4  8      0.6153846

 1    5  5      0.3846154

Answer 1

To print relative proportions, you can add another column that calculates relative frequencies using the mutate function from the dplyr package  as follows:

library(dplyr)

data(mtcars)

mtcars <- tbl_df(mtcars)

mtcars %>%

  group_by(am, gear) %>%

  summarise (n = n()) %>%

  mutate(freq = n / sum(n))

Output:

# A tibble: 4 x 4

# Groups:   am [2]

     am  gear     n  freq

  <dbl> <dbl> <int> <dbl>

1     0     3    15 0.789

2     0     4     4 0.211

3     1     4     8 0.615

4     1     5     5 0.385

To print the relative percentages:

mtcars %>%

  group_by(am, gear) %>%

  summarise (n = n()) %>%

  mutate(rel.freq = paste0(round(100 * n/sum(n), 0), "%"))

Output:

# A tibble: 4 x 4

# Groups:   am [2]

     am  gear     n rel.freq

  <dbl> <dbl> <int> <chr>   

1     0     3    15 79%     

2     0     4     4 21%     

3     1     4     8 62%     

4     1     5     5 38%