How to Perform Bartlett’s Test in Python

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In the realm of statistics and data analysis, Bartlett’s Test is a widely-utilized test that is used to check the homogeneity of variances among different groups. Essentially, it evaluates if multiple datasets come from populations with equal variances. The test assumes that the data is normally distributed. In this article, we will dive into the depths of Bartlett’s Test, understand its significance, and learn how to perform it using Python.

Background Knowledge

a. Variance

Variance is a statistical measure that tells us how much individual data points in a distribution deviate from the mean. In simpler terms, it is the average of the squared differences from the mean.

b. Homogeneity of Variances

Homogeneity of variances means that different groups or samples have the same variance. In many statistical tests, such as ANOVA, the assumption of equal variances is crucial for the validity of the results.

c. The importance of Checking Variance Homogeneity

Checking for homogeneity of variance is essential because many statistical tests assume that variances are equal across groups or samples. If this assumption is violated, the results may not be reliable.

Understanding Bartlett’s Test

a. Hypotheses

The hypotheses for Bartlett’s test are:

  • Null Hypothesis (H0): The variances are equal across all groups.
  • Alternative Hypothesis (H1): At least one group has a different variance.

b. Test Statistic

Bartlett’s test statistic is calculated based on the natural logarithm of the sample variances. Without going into the mathematical details, it’s important to understand that the test statistic follows a chi-squared distribution.

c. Assumptions

  • The samples are randomly drawn.
  • The data is normally distributed.

d. Applications

  • Preparatory test for ANOVA
  • Comparing variances in different groups in quality control.

Loading and Preparing Data

Before you can perform the Bartlett’s test, you need to have some data to work on. You can load your data from a CSV file, excel, SQL database, or any other source. The pandas library can be used for loading and preparing data.

import pandas as pd

# Load data from a CSV file
data = pd.read_csv('your-data-file.csv')

Performing Bartlett’s Test in Python

a. Using scipy.stats

The scipy library provides the bartlett method for performing Bartlett’s test.

from scipy.stats import bartlett

# Sample data
group1 = [2, 5, 6, 7, 10]
group2 = [3, 5, 6, 8, 12]
group3 = [1, 3, 5, 6, 9]

# Perform Bartlett's test
test_statistic, p_value = bartlett(group1, group2, group3)

print(f"Test statistic: {test_statistic}")
print(f"P-value: {p_value}")

b. Interpreting the Results

The result includes a test statistic and a p-value. A common significance level is 0.05. If the p-value is below this threshold, you would reject the null hypothesis in favor of the alternative – indicating that there is significant evidence that at least one of the groups has a different variance. If the p-value is higher, you do not have enough evidence to reject the null hypothesis of equal variances.

Alternative to Bartlett’s Test

If the normality assumption is violated, Levene’s test is a better option. It is less sensitive to departures from normality. It can be used in a similar way to Bartlett’s test using scipy:

from scipy.stats import levene

test_statistic, p_value = levene(group1, group2, group3)

Practical Example

Let’s go through a practical example by analyzing a dataset. Assume you have a dataset of exam scores of students from three different classes and you want to check if the variances in scores are equal across these classes.

import pandas as pd
from scipy.stats import bartlett

# Sample data
data = {
    'Class1': [89, 90, 92, 88, 87, 85, 90, 92, 91, 89],
    'Class2': [76, 78, 80, 75, 77, 79, 78, 80, 81, 77],
    'Class3': [92, 95, 93, 94, 91, 92, 93, 94, 96, 92]

# Convert to DataFrame
df = pd.DataFrame(data)

# Perform Bartlett's test
test_statistic, p_value = bartlett(df['Class1'], df['Class2'], df['Class3'])

# Output the results
print(f"Test statistic: {test_statistic}")
print(f"P-value: {p_value}")

# Interpret the results
alpha = 0.05
if p_value < alpha:
    print("Reject the null hypothesis - Variances are not equal across groups")
    print("Fail to reject the null hypothesis - Variances are equal across groups")


Bartlett’s Test is an essential tool for verifying the homogeneity of variances across multiple groups. Python, with its powerful libraries such as scipy, provides an efficient way to perform Bartlett’s Test. It is imperative to understand the underlying assumptions and hypotheses before interpreting the results. In cases where the normality assumption is not met, alternatives like Levene’s test should be considered.

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