Learn Randomization | A/B Testing Foundations

Randomization is the foundation of reliable A/B testing. When you compare a control group with a treatment group, you must assign users to these groups in a way that is free from patterns or biases. Without randomization, differences between groups could be caused by factors unrelated to your test—such as demographics, user behavior, or timing—instead of the variable you are studying.

Randomization helps ensure that both groups are statistically similar at the start of the experiment. This makes it much more likely that any observed differences in outcomes are due to your intervention, not hidden confounding variables.

You can use several methods to achieve randomization in A/B testing:

Simple random assignment: Each user has an equal chance of being placed in either the control or treatment group; this can be done using random number generators or hashing user identifiers;
Stratified randomization: Certain characteristics (like age or location) are balanced across groups by randomizing within strata.

For most digital experiments, simple random assignment using programming tools is both effective and efficient.


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import pandas as pd
import numpy as np

# Example user dataset
users = pd.DataFrame({
    "user_id": range(1, 11)
})

# Set random seed for reproducibility
np.random.seed(42)

# Randomly assign each user to 'control' or 'treatment'
users["group"] = np.random.choice(["control", "treatment"], size=len(users))

print(users)

By assigning users to groups at random, you reduce the risk of systematic bias in your experiment. This means that, on average, both groups will be similar in all characteristics except for the treatment itself. As a result, any difference in outcomes between the control and treatment groups can be more confidently attributed to the change you are testing, rather than to pre-existing differences or external influences. Randomization is therefore essential for drawing valid, trustworthy conclusions from A/B tests.

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Section 3. Chapter 3

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You can use several methods to achieve randomization in A/B testing:

Simple random assignment: Each user has an equal chance of being placed in either the control or treatment group; this can be done using random number generators or hashing user identifiers;
Stratified randomization: Certain characteristics (like age or location) are balanced across groups by randomizing within strata.

For most digital experiments, simple random assignment using programming tools is both effective and efficient.


              123456789101112131415
            
import pandas as pd
import numpy as np

# Example user dataset
users = pd.DataFrame({
    "user_id": range(1, 11)
})

# Set random seed for reproducibility
np.random.seed(42)

# Randomly assign each user to 'control' or 'treatment'
users["group"] = np.random.choice(["control", "treatment"], size=len(users))

print(users)

Everything was clear?

Thanks for your feedback!

Section 3. Chapter 3