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All about subject line split testing

Test up to three subject lines with a portion of your audience, and let us send the best-performing one to the remainder of your recipients. It's a surefire way to get more opens on each send-off and learn about your audience along the way.

How to send a subject line split test

With any mailing in our drag & drop editor, you have the option to conduct a subject line split test. Here's how to set one up: 

  1. Head to the Review & Send page, and click the Switch to split test button below the Enter a subject line field.
  2. Type in your variations. You can enter up to three, but you need at least two.
  3. Select whether you want to autosend the winning variation to the remaining 80% of your recipients when the test completes (which is what we recommend).
  4. Click Start split test now to start the split test right away, or Schedule split test to start later to launch the test at a designated time in the future.
  5. Watch the results tally in your Response section on a tab labeled Split test.

We will send those variations to a small subset of your audience (20%) to see which one gets the best open rate.

The split test will run for four hours. We recommend you leave the Austosend box checked so we will automatically send the best-performing subject line to the remainder of your recipients once the test completes.

However, if you wish to end the test early and send manually, you'll have the option to do so once the test is launched.

5 best practices for subject line split testing

The online conversion experts at Brooks Bell Interactive offer up five tips for setting up effective subject line split tests in your email marketing campaigns.

Email marketers who consistently conduct split tests know that the rewards are numerous—higher open rates, increased conversions and, perhaps most importantly, insights into subscriber behavior and preferences. But if you’re new to testing, how do you know you’re doing it right?

Testing subject lines is a simple, effective way to start building a solid email testing program. These five tips will help you get started today:

Get scientific. 

As you probably remember from seventh grade science, any good scientific experiment follows structured steps.  First, start with observation and research. For email marketing, this means analyzing data and reviewing current subject lines. Then, come up with a hypothesis based on your research. For example, your hypothesis might be, “A short subject line will lead to a higher open rate than a long subject line.” After you conduct the test, you’ll see if the results proved or disproved your hypothesis.

Identify goals.

There are many ways to measure success—how will you measure yours? Determining your business goals at the outset will help ensure that your testing mailing will be worth your time. Figure out which metrics you’ll be looking at once the test deploys. Open rate may indicate reader interest, but your ultimate goal may be to increase the number of people who submit the form on your landing page. In that case, the email version that gets the most opens may not be the one that leads to the most conversions.

Batch test first.

Conducting your test on a small sample of your subscriber list can be a smart way to maximize results. Use the winning subject line for the remainder of your subscriber list. Be aware, though, that batch testing only works when your results can reach statistical significance. The sample has to be large enough to draw sound conclusions; if your subscriber list is small, you won’t benefit from batch testing.

Test one variable.

Effective testing leads to learnings about your audience. But learnings aren’t clear when you test more than one variable at a time, because you aren’t able to tell which one made a difference. For example, you may want to test personalization: Will your readers respond better to a subject line that includes their first name? In that case, make sure the two subject lines you’re testing, aside from the personalization, are completely identical. Additional variations—whether in messaging, tone, length or offer—will lead to inconclusive results. Likewise, testing subject lines means that all other content is the same, from the email to the landing page.

Be consistent.

Contrary to what many email marketers believe, creating a compelling subject line isn’t hard. The real challenge is creating a compelling one that is consistent with your email and landing page. Otherwise, your awesome subject line could inspire a lot of opens, but very few clickthroughs. Worse, the email will likely end up in subscribers’ “deleted items” folder—and your subsequent emails will be regarded with suspicion. Take the long view when it comes to testing. Short-term wins are pointless if you’re not learning about your audience.


These five best practices are a good starting point for any email marketing mailing. But this is just the beginning. Testing is a mindset, not a tactic. Your results will help shape future tests, inspire additional testing ideas and form new research questions. After all, a testing expert's work is never done.

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