January 26, 2024

The Power of A/B Testing Subject Lines in Email Marketing

Are your email subject lines not getting the attention they deserve? Are you struggling to improve open rates and engagement with your email campaigns? Look no further than the power of A/B testing. 

Under the guidance of an expert digital agency like Schaefer, A/B testing subject lines is a game-changer, allowing you to optimize your email performance to determine the highest converting keywords and phrases, in order to drive up open and click rates and increase conversions. 

What is A/B Testing?

A/B testing, also known as split testing, is an invaluable technique in marketing that involves testing two different versions of content on an audience. These versions, labeled “version A” and “version B,” are nearly identical except for one key detail: a subject line, a call-to-action, an image, the sender’s name, an element of the email content, or even the time of sending. 

Start with Subject Lines

“If people are unwilling to open the emails you send them, the rest of your marketing campaign is simply going to fall flat.”


Subject lines are a great starting point in implementing A/B testing in your marketing efforts, as it provides an opportunity to test different theories about what encourages your contacts to open your emails. Additionally, since you’re testing on your actual audience, your expert agency can then utilize the personalized insights to optimize your future campaigns. 

Here are some ideas of what can be tested in a subject line split test:

  • Length
    • Does a shorter or longer subject line length perform better?
  • Personalization
    • Does including the contact’s first name resulted in improved metrics? Does it have no discernible effect?
  • Specific Word Inclusions
    • How does including your company name, core values, specific products or services, or other key words affect the results of the email?
  • Point of View
    • Does your audience respond to first-person, second-person, or third-person subject lines better?
  •  Sentence case vs. Title Case
    • How does capitalization affect your email metrics? 
  • Offer
    • What happens if you tease the inclusion of a discount code within the email, in the subject line?

You may even lean on the assistance of AI like Klaviyo’s Subject Line Assistant or Jasper.ai for subject line generation. When used under the guidance of an experienced marketing partner, this tool can increase efficiency and speed in conducting an A/B test on subject lines.

Analyzing the Results

After an A/B test concludes, it is important to analyze the data and provide recommendations. A skilled agency partner possesses the expertise to identify changes in the essential metrics and apply the results to future emails, for improved contact engagement and customer growth. 

Your expert partner will focus on the following key points:

  1. Focus on a couple of key metrics.  Trying to analyze too many metrics can lead to many different conclusions. Hone in on the results that were likely to be impacted by the A/B subject line test – open and clickrates:
    1. Open rate – the number of emails opened divided by the total number of emails delivered, excluding bounces or other delivery failures.
    2. Click rate – the number of unique clicks on a link divided by the total number of delivered emails, excluding bounces or other delivery failures.
      1. Since recent privacy law changes have affected the accuracy of open rate, click rate is another way to see the people that actually opened an email – by looking at the click rate.
  2. Retest, and retest again. Even a statistically significant result could be a false positive. Test it again to see if the results can be mimicked. 
  3. Patience. Email marketing is composed of longer results cycles than other types of digital advertising (like display ads). Consider running the test multiple times, and waiting up to seven days after an email is sent before compiling results.

A/B testing subject lines offers powerful potential benefits, like decreasing bounce rate, increasing open/click rate, and increasing user engagement. However, this process takes time – from setting up the test, to running it, to reviewing the results, and finally to implementing those insights into future tests. To fully leverage the advantages possible from A/B testing subject lines, it is crucial to collaborate with an expert marketing agency. 

At Schaefer, we strategically analyze the results of our A/B test variations to gain crucial insights into what resonates best with each individual client’s specific subscribers, and then use that knowledge to improve the overall effectiveness of future messaging, in order to drive business growth.