Often in statistics, the datasets we’re working with include categorical variables.

These are variables that take on names or labels. Examples include:

- Marital status (“married”, “single”, “divorced”)
- Smoking status (“smoker”, “non-smoker”)
- Eye color (“blue”, “green”, “hazel”)
- Level of education (e.g. “high school”, “Bachelor’s degree”, “Master’s degree”)

When fitting machine learning algorithms (like linear regression, logistic regression, random forests, etc.), we often convert categorical variables to **dummy variables**, which are numeric variables that are used to represent categorical data.

For example, suppose we have a dataset that contains the categorical variable **Gender**. To use this variable as a predictor in a regression model, we would first need to convert it to a dummy variable.

To create this dummy variable, we can choose one of the values (“Male”) to represent 0 and the other value (“Female”) to represent 1:

**How to Create Dummy Variables in Pandas**

To create dummy variables for a variable in a pandas DataFrame, we can use the pandas.get_dummies() function, which uses the following basic syntax:

**pandas.get_dummies(data, prefix=None, columns=None, drop_first=False)**

where:

**data**: The name of the pandas DataFrame**prefix**: A string to append to the front of the new dummy variable column**columns**: The name of the column(s) to convert to a dummy variable**drop_first**: Whether or not to drop the first dummy variable column

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

**Example 1: Create a Single Dummy Variable**

Suppose we have the following pandas DataFrame:

import pandas as pd #create DataFrame df = pd.DataFrame({'income': [45, 48, 54, 57, 65, 69, 78], 'age': [23, 25, 24, 29, 38, 36, 40], 'gender': ['M', 'F', 'M', 'F', 'F', 'F', 'M']}) #view DataFrame df income age gender 0 45 23 M 1 48 25 F 2 54 24 M 3 57 29 F 4 65 38 F 5 69 36 F 6 78 40 M

We can use the **pd.get_dummies()** function to turn gender into a dummy variable:

#convert gender to dummy variable pd.get_dummies(df, columns=['gender'], drop_first=True) income age gender_M 0 45 23 1 1 48 25 0 2 54 24 1 3 57 29 0 4 65 38 0 5 69 36 0 6 78 40 1

The gender column is now a dummy variable where:

- A value of
**0**represents “Female” - A value of
**1**represents “Male”

**Example 2: Create Multiple Dummy Variables**

Suppose we have the following pandas DataFrame:

import pandas as pd #create DataFrame df = pd.DataFrame({'income': [45, 48, 54, 57, 65, 69, 78], 'age': [23, 25, 24, 29, 38, 36, 40], 'gender': ['M', 'F', 'M', 'F', 'F', 'F', 'M'], 'college': ['Y', 'N', 'N', 'N', 'Y', 'Y', 'Y']}) #view DataFrame df income age gender college 0 45 23 M Y 1 48 25 F N 2 54 24 M N 3 57 29 F N 4 65 38 F Y 5 69 36 F Y 6 78 40 M Y

We can use the **pd.get_dummies()** function to convert gender and college both into dummy variables:

#convert gender to dummy variable pd.get_dummies(df, columns=['gender', 'college'], drop_first=True) income age gender_M college_Y 0 45 23 1 1 1 48 25 0 0 2 54 24 1 0 3 57 29 0 0 4 65 38 0 1 5 69 36 0 1 6 78 40 1 1

The gender column is now a dummy variable where:

- A value of
**0**represents “Female” - A value of
**1**represents “Male”

And the college column is now a dummy variable where:

- A value of
**0**represents “No” college - A value of
**1**represents “Yes” college

**Additional Resources**

How to Use Dummy Variables in Regression Analysis

What is the Dummy Variable Trap?

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