Quartiles | Statistics

Quartiles is a method to describe the spread in a distribution. Quartiles divide the distribution into four equal parts.

Quartiles involves a breakdown of values in proportions of 25 %. Before you make a division into quartiles, the observations have to be sorted from the lowest to the highest value, or vice versa. A division into quartiles results in three quartiles, Q1, Q2 and Q3. Q2 corresponds to the median in a distribution and has 50 % of the observations on each side of it.

Quartiles are calculated when the observations have been sorted from the lowest to the highest value and in accordance with the formula below.

Localization of quartiles:
Q1 = (n + 1)(25/100)
Q2 = (n + 1)(50/100)
Q3 = (n + 1 )(75/100)

where:
n = number of observations in the distribution.

Localization of quartiles means that you get a number for "n", you have to find the position in the distribution of observations and get the value that corresponds to each quartile. If the position is between two values, you have to calculate the value of the quartile with these two values and the position.