Contingency tables in R

Problem

We want to create a contingency table for one or multiple variables.

Solution

• One variable

table(mtcars$am)

 0  1 
19 13 

• Two variables

table(mtcars$am, mtcars$gear)

     3  4  5
  0 15  4  0
  1  0  8  5

If we want to include the names of the variables:

table(mtcars[, c("am", "gear")]) 
tabla <- table(mtcars[, 9:10])
# or the argument dnn:
table(mtcars$am, mtcars$gear, dnn = c("am", "gear"))

   gear
am   3  4  5
  0 15  4  0
  1  0  8  5

• Three variables

table(mtcars[, c("am", "gear", "cyl")])

, , cyl = 4

   gear
am   3  4  5
  0  1  2  0
  1  0  6  2

, , cyl = 6

   gear
am   3  4  5
  0  2  2  0
  1  0  2  1

, , cyl = 8

   gear
am   3  4  5
  0 12  0  0
  1  0  0  2

• Flat contingency tables

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

ftable(mtcars[, c("am", "gear", "cyl")])

        cyl  4  6  8
am gear             
0  3         1  2 12
   4         2  2  0
   5         0  0  0
1  3         0  0  0
   4         6  2  0
   5         2  1  2

We use the arguments row.vars and col.vars to provide the numbers or names of the variables to be used for the rows and columns of the flat contingency table. If neither of these two is given, the last variable is used for the columns. In our example the variable cyl.

ftable(mtcars[, c("am", "gear", "cyl")], col.vars = c(1, 2))

     am    0        1      
    gear  3  4  5  3  4  5
cyl                       
4         1  2  0  0  6  2
6         2  2  0  0  2  1
8        12  0  0  0  0  2

Alternative

The function xtabs creates contingency tables using a formula interface, each variable separated by +.

# One variable
xtabs(~ am, mtcars)
# Two variables
xtabs(~ am + gear, mtcars)
# Three variables
xtabs(~ am + gear + cyl, mtcars)
# Flat contingency table
ftable(xtabs(~ am + gear + cyl, mtcars))

Related posts

• Spanish version
• Proportion tables in R