A/B Testing Unveiled: Discovering the Ideal Subject Lines

Created On
August 31, 2023
Analyzing How A/B Testing Can Help Find the Best-Performing Subject Lines

Do almost 77% of marketers believe that email marketing is the perfect channel to increase the ROI of the business? 

Along with email marketing, several other aspects or best practices also need to be considered to get the best results from email marketing campaigns. A/B testing is one of these fantastic best practices most commonly used. This aspect helps skyrocket conversion rates and get valuable customers for the business. So let us move ahead in this article and learn a lot more insights and benefits of this simple process known as A/B testing and how it can help in finding the eye-catchers of any email that is the subject lines. 

The Psychology Behind Email Subject Lines

Email subject lines are the most essential aspect of email marketing. Because it is the prime opportunity for businesses to target their potential customers and indirectly convince them to open their marketing email, business owners need to understand the psychology behind the email subject lines, as they are the power of the businesses to capture their recipient's attention. So the business should follow some of the below simple best practices for engaging the customers:

  • Keep it short: The optimum length of an email subject line should be either 35 characters or less. Because most people check out their emails on their mobile phones these days, the entire email subject line should be easily visible and readable on the mobile screen itself. 
  • Include some eye-catching words: Eye-catching words are the words that guarantee the attention of your readers. Some of the examples of such words are: Thank you, New, Free. Including such attractive words in your email subject lines increases your email open rate. 
  • Personalization: Starting the subject line with some personalization, like the recipient's name, is another hit for the email subject lines. You can also refer to Smartlead, the most renowned email marketing software, for crafting unique subject lines for the fantastic response rates on your marketing emails.
  • Open-ended questions: Try to include open-ended questions in the email subject lines as that increases the curiosity in the readers to open and read the emails.
  • Add some numbers: Looking at numbers in the subject lines looks appealing to the readers.

Understand the Process of A/B Testing

A/B testing is the process of showing two different versions of the same website of any business to the website visitors at the same time to determine further which version is performing well and getting more visitors. 

It simply confirms which version performs better, like version A or B. Therefore, it is also known as split testing, which is like an experiment in two different versions of any marketing email, website, or advertisement shown to the visitors to determine which one gets more attention and more conversions. Businesses usually prefer doing A/B testing when they want to increase the number of visitors to their websites. 

When you perform A/B testing on your marketing emails, that helps in improving the metrics of the same such as:

  • Improves the email open rate,
  • Improves the mail click-through rate, and
  • Improves the conversion rate and the sales of the business.

Insights About A/B Testing of Email Subject Lines

The email subject line is the most popular element of the marketing emails to be tested. The common reason behind this is that changing or writing different versions of an email subject line is easy. Even businesses have like a lot of ideas to write potential subject lines. 

So the businesses start their email marketing campaign with two different subject lines and then keep an eye on the email open rate metric of the campaign to review the results of this test. When the recipients view and open your emails, you know how many clicked upon the links inside them. 

The email open rate is the first metric affected by A/B testing with the email subject lines. Because when the most attractive and engaging subject line catches the reader's attention, they open the email. Further, the content and design of the email should also be interesting, forcing the reader to click on the links included in the email, improving the click-through and conversion rates. 

Best Practices for A/B Testing Subject Lines

To maximize the effectiveness of A/B testing for subject lines, consider these best practices:

Testing of Variables

To determine the impact of the subject line on the readers, ensure that you keep the other content of the email the same in both variables while performing the A/B testing. This step can help you determine which subject line variable helped you get more email open rates.

Make a Power Group

This power group will act like a control group for your campaign, where you can receive all the previous and existing subject line examples. This can help you create a baseline to compare the various subject lines and make appropriately effective changes. 

Divide Your Target Audience

The point behind these best practices is that everybody has a different way of thinking so different audiences can react differently to your subject lines. So, it would be best to segment or divide your target audience based on their behavior, location, demographics, and many other relevant factors. You can perform A/B testing on all the segmented audiences to get effective results.

Send Test Emails to a Larger Audience

Ensure that you are sending testing subject lines to a larger group of people so that more people can respond to significantly monitor the results of this A/B testing. 

Keeping a Track

Ensure to perform A/B testing for longer durations because tracking the impact of subject lines for a longer run is beneficial. 

Document Everything

Starting from the examples, results, and lessons learned from those tests, document every progress and information about the A/B tests performed.

Insights About the A/B Testing Process

Specifically for email and marketing campaigns, the A/B testing process involves the creation of two different versions of emails to be sent to the target audience, one with some specific variation of the subject line and another with another variant of the subject line. These two variations are then forwarded to the segmented audience, and the version that performs better and gives good results is preferred. Some of the essential steps included in this process of A/B testing of email subject lines are:

  • Creating test groups

Two test groups are created to perform A/B testing. Group A and Group B. Group A receives one variant of an email featuring its subject line, and Group B gets a similar email with just a different version of the subject line. Ensure that these groups are selected randomly to monitor the result appropriately. 

  • Collection of data

Once the recipients receive the two different versions of emails, the data is collected on how each variant has performed based on:

  • Email open rate
  • Click-through rate
  • The conversion rate
  • Analysis based on statistics

This analysis is the central part of the A/B testing process because it helps identify the differences in the performance of both the subject line versions. 

  • Future improvement

The A/B testing process is not just for one end performance, though the results from this testing also help the marketers use those learning in future email marketing strategies. 

  • Personalization

A/B testing also depends greatly on the personalization level in the email subject lines. Marketers can typically experiment to create different versions of email subject lines and add a personal touch, like adding the recipient's names in the subject lines. This technique attracts the readers and lets them know that the email is tailored for a specific audience. 

How do You Perform an A/B Test on a Subject Line?

Performing an A/B test on a subject line for an email marketing campaign involves systematically comparing two subject line variations and determining which one performs better. Here's a step-by-step guide on how to conduct an A/B test on a subject line:

Step 1: Define Your Objective and Hypothesis

  • Identify Your Goal: Determine what you want to achieve with your email campaign. Do you aim to increase open rates, click-through rates, conversions, or other metrics?
  • Formulate a Hypothesis: Create a clear hypothesis regarding how a particular change in the subject line will impact your goal. For example, "I hypothesize that a subject line with a sense of urgency will lead to higher open rates."

Step 2: Segment Your Email List

Divide your email list into two or more segments, depending on the number of variations you want to test. In this case, you'll have a control group and one or more test groups. Ensure that the segments are randomly selected and statistically significant in size.

Step 3: Create Subject Line Variations

Develop the subject line variations you want to test. Typically, you'll have a control group with the original subject line (the current standard) and one or more test groups with different subject line variations. Variations can include changes in wording, length, tone, personalization, or any other element you want to test.

Step 4: Set Up Your Email Campaign

Now, set up your email campaign, making sure to follow these guidelines:

  • Send Time: Choose a consistent time for all variations to avoid time-related biases.
  • Content Consistency: Keep the email content consistent across variations to isolate the impact of the subject line.
  • Tracking: Ensure you have proper tracking mechanisms to monitor open rates, click-through rates, and other relevant metrics.

Step 5: Send the Emails

Send the email variations to their respective segments, ensuring the only difference between them is the subject line. This minimizes external factors that could affect the results.

Step 6: Collect Data

After the emails have been sent and recipients have had time to engage with them, collect data on the performance of each subject line variation. Key metrics to monitor include:

  • Open Rates: The percentage of recipients who opened the email.
  • Click-Through Rates (CTR): The percentage of recipients who clicked on links within the email.
  • Conversion Rates: The percentage of recipients completing a desired action (e.g., purchasing) after opening the email.

Step 7: Analyze the Results

Perform statistical analysis to determine which subject line variation performed better. Standard statistical tests for this analysis include t-tests or chi-squared tests, depending on the data type (continuous or categorical).

Consider the following factors when analyzing the results:

  • Statistical Significance: Determine whether the differences in open rates, CTR, or other metrics are statistically significant. A p-value of less than 0.05 is often considered statistically significant.
  • Effect Size: Assess the practical significance of the differences. Even if a difference is statistically significant, it may not be substantial enough to warrant a change in strategy.
  • Confidence Intervals: Calculate confidence intervals to understand the range within which the proper population parameter lies.

Step 8: Make Informed Decisions

Based on the analysis, make informed decisions about which subject line variation to use for future email campaigns. Choose the subject line that aligns with your campaign objectives and demonstrates superior performance in the A/B test.

Step 9: Document and Iterate

Document your A/B test results, including the subject line variations, performance metrics, and the decision made. Use this information to refine your email marketing strategy and inform future A/B tests.

Step 10: Implement and Monitor

Implement the winning subject line variation in your email campaign. Continue to monitor performance to ensure that the improvements observed in the A/B test translate into sustained results.

How do you Analyze A/B Test Results?

Analyzing A/B test results is crucial in determining which variation (A or B) of your experiment performs better and should be implemented in your campaign or website. Proper analysis ensures that your decisions are data-driven and statistically sound. Here is a step-by-step guide on how to analyze A/B test results:

  • Collect Data

Before you can analyze the results, you need to gather relevant data from both versions of your test (A and B). This data typically includes metrics like click-through rates, conversion rates, revenue, engagement, or any other relevant KPIs you are measuring.

  • Define Your Primary Metric

Determine the primary metric you will use to evaluate the success of your test. This metric should align with your experiment's goal. For example, if your goal is to increase email click-through rates, your primary metric might be the percentage of recipients who clicked on a link in the email.

  • Calculate

Calculate the difference between the two variations (A and B) for your chosen primary metric. You can do this by subtracting the metric value of variation A from variation B. This provides an initial understanding of how the variations differ.

  • Statistical Significance Testing

Determining whether the differences you observe are statistically significant or could have occurred by chance is essential. Standard statistical tests for A/B testing include t-tests and chi-squared tests, depending on the data type you are analyzing (continuous or categorical).

  • T-Tests: These are used for continuous data like conversion rates. They help determine if the difference between the two groups is statistically significant.
  • Chi-Squared Tests: These are used for categorical data, such as the number of users who clicked a button.
  • Calculate Confidence Intervals

Calculate the confidence intervals for both variations. Confidence intervals provide a range within which you can be confident that the valid population parameter lies. If the confidence intervals for the two interpretations do not overlap, this is a strong indicator of statistical significance.

  • Effect Size

Consider the effect size, which tells you the difference's practically significant. Even if a difference is statistically significant, it might not be substantial. A small effect size may not warrant a change in your strategy.

  • Sample Size

Ensure that your sample sizes are adequate. Small sample sizes can lead to unreliable results. Use power calculations before conducting your test to determine the required sample size for statistical significance.

  • Segmentation

Sometimes, the overall results mask valuable insights. Consider segmenting your data to see if certain groups or demographics respond differently to the variations. This can help you tailor your strategies more effectively.

  • Monitoring Over Time

If your experiment runs for an extended period, monitor the results regularly. Sometimes, effects can change over time, and this information can be essential for decision-making.

  • Draw Conclusions

Based on the statistical significance, confidence intervals, p-values, effect sizes, and other relevant factors, determine which variation performed better. Decide whether to implement the changes suggested by the winning variation.

  • Document and Communicate

Document your findings and communicate them to relevant stakeholders. It's essential to have a clear record of the analysis and the decisions made based on the results.

  • Iterate

A/B testing is an iterative process. Use the insights from one test to inform future experiments and refine your strategies.


In the dynamic world of email marketing, subject lines remain a linchpin for success. A/B testing serves as the compass that guides marketers in their quest to discover subject lines that resonate with their audience and drive engagement. By formulating hypotheses, conducting controlled experiments, and analyzing data rigorously, marketers can unlock the secrets to crafting subject lines that maximize open rates, click-through rates, conversions, and, ultimately, the success of their email campaigns. Remember, the journey to finding the perfect subject line is ongoing. Still, with A/B testing as your trusty companion, you're well-equipped to navigate the ever-evolving terrain of email marketing and unearth the subject lines that yield the most significant rewards.

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Author’s Details

Tanmay Shukla

Tanmay is a SEO Content Writer at Smartlead. His zeal and enthusiasm in writing led him to the path of being a Content Writer. He has been advancing in this field by possessing more knowledge and skills. His constant endeavor has made him a whiz in SEO, Creative Writing, and driving sales through writing.


Edited by:

Charu Mitra Dubey


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