How to create tertile in R

Question

I Have a column in my dataframe called Score for example

DF$Score<-(1.2,2,2,3.2,4.4,4.5,2.5,6.7,8.9,4.8)

I want to make a new column containing tertiles of this column (called Low, Medium and High). How to do that in R? I knoe quantile function but i am asking about tertiles here.

Answer 1

DF = data.frame(Score = c(1.2,2,2,3.2,4.4,4.5,2.5,6.7,8.9,4.8))

# Find tertiles
vTert = quantile(DF$Score, c(0:3/3))

# classify values
DF$tert = with(DF, 
               cut(Score, 
                   vTert, 
                   include.lowest = T, 
                   labels = c("Low", "Medium", "High")))

# The result
> DF
   Score   tert
1    1.2    Low
2    2.0    Low
3    2.0    Low
4    3.2 Medium
5    4.4 Medium
6    4.5 Medium
7    2.5    Low
8    6.7   High
9    8.9   High
10   4.8   High

Answer 2

Another option would be to use dplyr:

df %>%
  mutate(tertiles = ntile(Score, 3)) %>%
  mutate(tertiles = if_else(tertiles == 1, 'Low', if_else(tertiles == 2, 'Medium', 'High'))) %>%
  arrange(Score)

# Score tertiles
# 1    1.2      Low
# 2    2.0      Low
# 3    2.0      Low
# 4    2.5      Low
# 5    3.2   Medium
# 6    4.4   Medium
# 7    4.5   Medium
# 8    4.8     High
# 9    6.7     High
# 10   8.9     High