Create a Factor Variable Using the Quantiles of a Continuous Variable

Creates a factor variable using the quantiles of a continuous variable.

Usage

cutQ(
  x,
  breaks = quantile(x, seq(0, 1, by = 0.25), na.rm = TRUE),
  labels = NULL,
  na.rm = FALSE,
  ...
)

Arguments

x: continuous variable
breaks: cut points used to create groups. By default, quartiles are used. See quantile() for details. A single integer specifies the intended number of groups; for example, breaks = 10 creates deciles.
labels: labels for the levels of the resulting category. By default, labels are defined as Q1, Q2, and so on. The argument is passed to cut(), so labels = FALSE returns integer codes instead of a factor. When quantiles are tied, the levels are built from the observed interval bounds instead and labels is ignored - a warning is issued in that case.
na.rm: logical. Should missing values be removed before the quantiles are computed? Defaults to FALSE. Note that the quantiles themselves are always taken with na.rm = TRUE; this argument decides whether the missing values are dropped from the returned factor as well.
...: optional arguments passed to cut()

Value

a factor with one level for each quantile interval, or an integer vector when labels = FALSE

Details

This function uses quantile() to obtain the specified quantiles of x, then calls cut() to create a factor variable using the intervals specified by these quantiles.

It properly handles cases where more than one quantile obtains the same value, as in the second example below. Note that in this case, there will be fewer generated factor levels than the specified number of quantile intervals.

Note

Based on code by Gregory R. Warnes, adapted to conform to package standards.

Examples


# create example data
set.seed(1234)
x <- rnorm(1000)

# cut into quartiles
quartiles <- cutQ(x)
table(quartiles)
#> quartiles
#>  Q1  Q2  Q3  Q4 
#> 250 250 250 250 

# cut into deciles
deciles <- cutQ(x, breaks=10, labels=NULL)
table(deciles)
#> deciles
#>  Q1  Q2  Q3  Q4  Q5  Q6  Q7  Q8  Q9 Q10 
#> 100 100 100 100 100 100 100 100 100 100 

# show handling of 'tied' quantiles.
x <- round(x)  # discretize to create ties
stem(x)        # display the ties
#> 
#>   The decimal point is at the |
#> 
#>   -3 | 0000000000000
#>   -2 | 
#>   -2 | 000000000000000000000000000000000000000000000000
#>   -1 | 
#>   -1 | 00000000000000000000000000000000000000000000000000000000000000000000+181
#>   -0 | 
#>   -0 | 
#>    0 | 00000000000000000000000000000000000000000000000000000000000000000000+310
#>    0 | 
#>    1 | 00000000000000000000000000000000000000000000000000000000000000000000+140
#>    1 | 
#>    2 | 000000000000000000000000000000000000000000000000000000000000000
#>    2 | 
#>    3 | 00000
#> 
deciles <- cutQ(x, breaks=10)

table(deciles) # note that there are only 5 groups (not 10) 
#> deciles
#> [-3,-1)      -1       0       1   (1,3] 
#>      61     261     390     220      68 
               # due to duplicates