How to Calculate Studentized Residuals in Python

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A studentized residual is simply a residual divided by its estimated standard deviation.

In practice, we typically say that any observation in a dataset that has a studentized residual greater than an absolute value of 3 is an outlier.

We can quickly obtain the studentized residuals of a regression model in Python by using the OLSResults.outlier_test() function from statsmodels, which uses the following syntax:

OLSResults.outlier_test()

where OLSResults is the name of a linear model fit using the ols() function from statsmodels.

Example: Calculating Studentized Residuals in Python

Suppose we build the following simple linear regression model in Python:

#import necessary packages and functions
import numpy as np
import pandas as pd
import statsmodels.api as sm
from statsmodels.formula.api import ols

#create dataset
df = pd.DataFrame({'rating': [90, 85, 82, 88, 94, 90, 76, 75, 87, 86],
                   'points': [25, 20, 14, 16, 27, 20, 12, 15, 14, 19]})

#fit simple linear regression model
model = ols('rating ~ points', data=df).fit()

We can use the outlier_test() function to produce a DataFrame that contains the studentized residuals for each observation in the dataset:

#calculate studentized residuals
stud_res = model.outlier_test()

#display studentized residuals
print(stud_res)

    student_resid	 unadj_p	 bonf(p)
0	-0.486471	0.641494	1.000000
1	-0.491937	0.637814	1.000000
2 	 0.172006	0.868300	1.000000
3	 1.287711	0.238781	1.000000
4	 0.106923	0.917850	1.000000
5	 0.748842	0.478355	1.000000
6	-0.968124	0.365234	1.000000
7	-2.409911	0.046780	0.467801
8	 1.688046	0.135258	1.000000
9	-0.014163	0.989095	1.000000

This DataFrame displays the following values for each observation in the dataset:

• The studentized residual
• The unadjusted p-value of the studentized residual
• The Bonferroni-corrected p-value of the studentized residual

We can see that the studentized residual for the first observation in the dataset is -0.486471, the studentized residual for the second observation is -0.491937, and so on.

We can also create a quick plot of the predictor variable values vs. the corresponding studentized residuals:

import matplotlib.pyplot as plt

#define predictor variable values and studentized residuals
x = df['points']
y = stud_res['student_resid']

#create scatterplot of predictor variable vs. studentized residuals
plt.scatter(x, y)
plt.axhline(y=0, color='black', linestyle='--')
plt.xlabel('Points')
plt.ylabel('Studentized Residuals') 

From the plot we can see that none of the observations have a studentized residual with an absolute value greater than 3, thus there are no clear outliers in the dataset.

Additional Resources

How to Perform Simple Linear Regression in Python
How to Perform Multiple Linear Regression in Python
How to Create a Residual Plot in Python