In statistics, **quantiles** are values that divide a ranked dataset into equal groups.

To calculate the quantiles grouped by a certain variable in R, we can use the following functions from the dplyr package in R:

library(dplyr) #define quantiles of interest q = c(.25, .5, .75) #calculate quantiles by grouping variable df %>% group_by(grouping_variable) %>% summarize(quant25 = quantile(numeric_variable, probs = q[1]), quant50 = quantile(numeric_variable, probs = q[2]), quant75 = quantile(numeric_variable, probs = q[3]))

The following examples show how to use this syntax in practice.

**Examples: Quantiles by Group in R**

The following code shows how to calculate the quantiles for the number of wins grouped by team for a dataset in R:

library(dplyr) #create data df <- data.frame(team=c('A', 'A', 'A', 'A', 'A', 'A', 'A', 'A', 'B', 'B', 'B', 'B', 'B', 'B', 'B', 'B', 'C', 'C', 'C', 'C', 'C', 'C', 'C', 'C'), wins=c(2, 4, 4, 5, 7, 9, 13, 13, 15, 15, 14, 13, 11, 9, 9, 8, 8, 16, 19, 21, 24, 20, 19, 18)) #view first six rows of data head(df) team wins 1 A 2 2 A 4 3 A 4 4 A 5 5 A 7 6 A 9 #define quantiles of interest q = c(.25, .5, .75) #calculate quantiles by grouping variable df %>% group_by(team) %>% summarize(quant25 = quantile(wins, probs = q[1]), quant50 = quantile(wins, probs = q[2]), quant75 = quantile(wins, probs = q[3])) team quant25 quant50 quant75 1 A 4 6 10 2 B 9 12 14.2 3 C 17.5 19 20.2

Note that we can also specify any number of quantiles that we’d like:

#define quantiles of interest q = c(.2, .4, .6, .8) #calculate quantiles by grouping variable df %>% group_by(team) %>% summarize(quant20 = quantile(wins, probs = q[1]), quant40 = quantile(wins, probs = q[2]), quant60 = quantile(wins, probs = q[3]), quant80 = quantile(wins, probs = q[4])) team quant20 quant40 quant60 quant80 1 A 4 4.8 7.4 11.4 2 B 9 10.6 13.2 14.6 3 C 16.8 18.8 19.2 20.6

We can also choose to calculate just one quantile by group. For example, here’s how to calculate the 90th percentile of the number of wins for each team:

#calculate 90th percentile of wins by team df %>% group_by(team) %>% summarize(quant90 = quantile(wins, probs = 0.9)) team quant90 1 A 13 2 B 15 3 C 21.9

**Additional Resources**

How to Calculate Quartiles in R

How to Calculate Deciles in R

How to Calculate Percentiles in R