Calculate Quartiles in Python

Renesh Bedre    2 minute read

Quartiles are values that divide a dataset into four equal parts, each containing 25% of the data. Quartiles are useful for understanding the spread and distribution of a dataset.

In general, there are three quartiles used. Q1 (first quartile), Q2 (second quartile), and Q3 (third quartile) are the values below which 25%, 50%, and 75% of the data fall.

calculate quartiles in Python

In Python, the quartiles can be calculated using the quantile() function from the NumPy and pandas package.

The general syntax of quantile() looks like this:

# calculate quartiles using using NumPy
import numpy as np
np.quantile(x, [0.25, 0.5, 0.75])

# calculate quartiles using pandas
import pandas as pd
df['col_name'].quantile([0.25, 0.5, 0.75])

Where, x is the dataset in array format and the second array is the probability for the quantiles to compute.

The following examples explain how to use the quantile() function from NumPy and pandas to calculate quartiles

Example 1: calculate quartiles using quantile() from NumPy

Suppose, you have a following dataset,

x = [5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60]

Calculate the quartiles using quantile() function from NumPy,

# import package
import numpy as np

# calculate quartiles
np.quantile(x, [0.25, 0.5, 0.75])

# output
array([18.75, 32.5 , 46.25])

The Q1, Q2, and Q3 quartile values are 18.75, 32.5, and 46.25, respectively.

Example 2: calculate quartiles using quantile() from pandas

Suppose, you have the following pandas DataFrame,

# import package
import pandas as pd

# create random pandas DataFrame
df = pd.DataFrame({'col1': ['A', 'B', 'C', 'D', 'E', 'F', 'G', 'H', 'I', 'J', 'K', 'L'],
                   'col2': [5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55, 60]})

# view first few rows
df.head(2)
  col1  col2
0    A     5
1    B    10

# calculate quartiles
df['col1'].quantile([0.25, 0.5, 0.75])

# output
df['col2'].quantile([0.25, 0.5, 0.75])
0.25    18.75
0.50    32.50
0.75    46.25
Name: col2, dtype: float64

The output shows that Q1, Q2, and Q3 quartile values are 18.75, 32.5 , and 46.25, respectively.

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