Crosstab and Stacked Bar Chart in Python

Analysis of Two Categorical Variables

Crosstab is the most preferred way of analyzing two categorical variables. It helps you to identify the affinity between the two categories. Graphically a Crosstab can be displayed using Stacked Bar Chart.

The Application of crosstab can be best understood by working on sample data. So let’s jump into an example.

Analysis of the MBA Data continued…

For crosstab, we require two categorical variables. Let us analyze the association between MBA Specialization chosen by students and their stream in 12th Standard. The fields in the data file are “mba_specialization” and “ten_plus_2_stream”.

Data Import in Python

# Import the required packages
import pandas as pd
import os
import matplotlib.pyplot as plt
import seaborn as sns

%matplotlib inline

# set directory as per your file folder path
os.chdir("d:/k2analytics/datafile")


# read the file
mba_df = pd.read_csv("MBA_Students_Data.csv")

Crosstab of MBA Specialization vs 12th Standard Stream

Before jumping into the creation of crosstab. Let us quickly do the univariate analysis of both fields.

# Frequency Distribution of MBA Specialization
mba_df["mba_specialization"].value_counts()

Finance               80
Marketing             70
Business Analytics    25
HR                    25
Name: mba_specialization, dtype: int64

# Frequency Distribution of 12th Standard Stream
mba_df["ten_plus_2_stream"].value_counts()

Commerce                             125
Science                               70
Commerce - Information Technology      1
Science - Maths & Biology              1
Diploma in Mechanical Engineering      1
Science - PCM                          1
Computer Science                       1
Name: ten_plus_2_stream, dtype: int64

Data cleaning of stream

We need to do some data cleaning. Recategorize the individual cases into Commerce / Science category.

mba_df['ten_plus_2_stream_recat'] = mba_df['ten_plus_2_stream'].apply(
    lambda x : "Commerce" if ("Commerce" in x ) else "Science")

mba_df["ten_plus_2_stream_recat"].value_counts()

Commerce                             126
Science                               74
Name: ten_plus_2_stream_recat, dtype: int64

Crosstab Analysis

pd.crosstab(mba_df["ten_plus_2_stream_recat"],
            mba_df["mba_specialization"],
            dropna = False,
            margins = True
           )

mba_specialization	Business Analytics	Finance	HR	Marketing	All
ten_plus_2_stream_recat
Commerce	13	57	21	35	126
Science	12	23	4	35	74
All	25	80	25	70	200

From the above crosstab, we observe that there are 57 Commerce students who have taken Finance and likewise, 23 Science students have taken Finance. We can make more such statements but it is not giving any significant insights as such. To get the insight we must convert these numbers into some proportions like Row Proportions or Column Proportions.

Row Proportions

pd.crosstab(mba_df['ten_plus_2_stream_recat'],
            mba_df['mba_specialization'],
            dropna = False,
            normalize = 'index' # convert absolute to row proportions
           ).round(3)*100

mba_specialization	Business Analytics	Finance	HR	Marketing
ten_plus_2_stream_recat
Commerce	10.3	45.2	16.7	27.8
Science	16.2	31.1	5.4	47.3

Column Proportions

pd.crosstab(mba_df['ten_plus_2_stream_recat'],
            mba_df['mba_specialization'],
            dropna = False,
            normalize = 'columns' # convert absolute to column proportions
           ).round(3)*100

mba_specialization	Business Analytics	Finance	HR	Marketing
ten_plus_2_stream_recat
Commerce	52.0	71.2	84.0	50.0
Science	48.0	28.8	16.0	50.0

Inferences / Take away

Converting the absolute values of crosstab into Row Proportions / Column Proportions provides us more insight into data.

47% of Science graduate students prefer to pursue an MBA in Marketing
45% of Commerce graduate students have an inclination to pursue MBA in Finance.
Only 5% of Science Graduate preferred to take HR Specialization in MBA

From the crosstab, it is very clear that Science Students prefer to do MBA Specialization in Marketing and Commerce Students prefer Finance.

Association Analysis

Association analysis enables you to identify items that have an affinity for each other. Crosstab helps you present the interrelation between the two variables in a structured way. Crosstab is also called Contingency Table.

A statistical test called the chi-square test for independence, also called the chi-square test of association, is used on the Crosstab data (Contingency Table) to discover if there is a statistically significant relationship between two categorical variables

Stacked Bar Plot

Crosstab provides a great deal of insight when analyzing two categorical variables. Visually we can represent the crosstab as a Stacked Bar Chart as shown below:

Practise Exercise

Analyze Gender vs Stream of the student in 12th Standard (“gender” variable vs “ten_plus_2_stream”).

Upcoming Blog

In the upcoming blog, we will learn about “Analysis of Two Variables – One Categorical and Other Continuous”

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