How to calculate the percent of column total in R

Problem

We want to calculate the percent of column total in R. In our example, the percent of column freq: 7/397, 23/397, etc.

    x freq
1 Jan    7
2 Feb   23
3 Mar   86
4 Apr  281

Data:

df <- read.table(text = "x    freq
                        Jan   7
                        Feb   23
                        Mar   86
                        Apr   281", 
                        header = TRUE)

Solution

We create the percent of column total using the function prop.table.

df$prob <- prop.table(df$freq)
# Percentages with two decimal places
df$prob <- round(prop.table(df$freq), 4)*100

    x freq  prob
1 Jan    7  1.76
2 Feb   23  5.79
3 Mar   86 21.66
4 Apr  281 70.78

If we'd like to calculate the percent of a specific row, February in our example:

prop.table(df$freq)[df$x == "Feb"] 

 [1] 0.05793451

Alternatives

• Base package

df$prob <- df$freq/sum(df$freq)

• dplyr

library(dplyr)
df %>% mutate(prob = prop.table(freq))
# Or
df %>% mutate(prob = freq / sum(freq))

A specific row:

df %>% filter(x == "Feb")

    x freq       prob
1 Feb   23 0.05793451

References

• Proportion tables in R
• Contingency tables in R
• Transforming contingency tables into frequency tables in R

Proportion tables in R

Problem

We want to create proportion tables for one or multiple variables.

Solution

• One variable

tabla <- table(mtcars$am)
prop.table(tabla)

      0       1 
0.59375 0.40625

• Two variables

tabla <- table(mtcars[, c("am", "gear")])
prop.table(tabla)

   gear
am        3       4       5
  0 0.46875 0.12500 0.00000
  1 0.00000 0.25000 0.15625

The prop.table function has two arguments:
• x, table created with the function table
• margin, with three possible values:
  Null - x/sum(x) default like in the previous example.
  1 - proportion calculated by rows.
  2 - proportion calculated by columns.


# By row
prop.table(tabla, 1)

   gear
am          3         4         5
  0 0.7894737 0.2105263 0.0000000
  1 0.0000000 0.6153846 0.3846154

# By column
prop.table(tabla, 2)

   gear
am          3         4         5
  0 1.0000000 0.3333333 0.0000000
  1 0.0000000 0.6666667 1.0000000

• Three variables

tabla <- table(mtcars[, c("am", "gear", "cyl")])
prop.table(tabla)

, , cyl = 4

   gear
am        3       4       5
  0 0.03125 0.06250 0.00000
  1 0.00000 0.18750 0.06250

, , cyl = 6

   gear
am        3       4       5
  0 0.06250 0.06250 0.00000
  1 0.00000 0.06250 0.03125

, , cyl = 8

   gear
am        3       4       5
  0 0.37500 0.00000 0.00000
  1 0.00000 0.00000 0.06250

• Flat Contingency Table

In the previous example, a better approach would be to create a flat contingency table..

tabla <- ftable(mtcars[, c("am", "gear", "cyl")])
prop.table(tabla)

        cyl       4       6       8
am gear                            
0  3        0.03125 0.06250 0.37500
   4        0.06250 0.06250 0.00000
   5        0.00000 0.00000 0.00000
1  3        0.00000 0.00000 0.00000
   4        0.18750 0.06250 0.00000
   5        0.06250 0.03125 0.06250

• Percentage table

We can use the function round.

round(prop.table(tabla)*100, 2)

         cyl     4     6     8
am gear                      
0  3         3.12  6.25 37.50
   4         6.25  6.25  0.00
   5         0.00  0.00  0.00
1  3         0.00  0.00  0.00
   4        18.75  6.25  0.00
   5         6.25  3.12  6.25

round(prop.table(tabla, 1)*100, 2) # By row, am y gear.

        cyl     4     6     8
am gear                      
0  3         6.67 13.33 80.00
   4        50.00 50.00  0.00
   5          NaN   NaN   NaN
1  3          NaN   NaN   NaN
   4        75.00 25.00  0.00
   5        40.00 20.00 40.00

round(prop.table(tabla, 2)*100, 2) # By column, cyl

        cyl     4     6     8
am gear                      
0  3         9.09 28.57 85.71
   4        18.18 28.57  0.00
   5         0.00  0.00  0.00
1  3         0.00  0.00  0.00
   4        54.55 28.57  0.00
   5        18.18 14.29 14.29

References

• Contingency tables in R