Split ‘Em Up: Tips for Email Subject Line Testing


David Luther

Author Bio

Digital content strategist, content outreach specialist, content writer, SEO expert, and hot sauce aficionado.

Red outfit or blue outfit?

You’re attending a big event and want to make a great impression from the moment you walk through the door. But you’re stuck on which outfit to wear. You ask for advice from your significant other. Maybe seek a second opinion from a friend – just to be safe.

And there you have it. You just did an A/B test.

The idea behind email subject line testing is no different. You want to make a great impression with an optimized subject line so your subscribers will open your email.

Regular A/B testing not only helps you gain a clear understanding of your subscribers, but also helps create a metric-based email marketing program that yields more opens, clicks, and conversions.

Why subject line testing is important

A subject line is a first impression. Even if you’re sending to engaged subscribers, the average person receives almost 100 emails a day between work and personal accounts, with nearly half of email recipients using the subject line to determine whether or not to open each email.

Your preheaders deserve special attention as well. Subject lines typically cut off after 40 characters on mobile devices, so preheaders give you more space to make your point. Make sure to test them too.

How to perform an email subject line test

An established, repeatable test routine helps identify the difference that affect open rates. These steps show you how to run a test and analyze your results.

Step 1: Determine your send list

Segment-based testing allows you to see which subject lines work best with a particular group without impacting your entire list.

Popular segments to test:

  • New vs. returning customers
  • Mobile vs. desktop openers
  • High average order value customers

Step 2: Choose what to test

There’s a variety of elements you can test in your subject lines.

Is brevity better? Research shows that 40-50 characters is the sweet spot, but if your event is in Albuquerque, then your subject lines may have to be a little longer.

What about personalization? Adding subscribers’ names to subject lines is a popular tactic. Using “you” can be effective as well – but make sure your data is accurate so your personalization doesn’t backfire. Julie shouldn’t receive an email with John’s name on it.

What should the subject line look like? You’ve seen emails WITH ALL CAPS. And those with 😍🔥🐐. Figure out what works for your brand.

Some other subject line elements to test:

  • Personalization by purchase history
  • Creating urgency
  • Making a statement vs. asking a question

Step 3: Analyze the A/B test results

Looking beyond open rates helps you determine the overall effectiveness of your email campaigns. It’s easy to determine a winner based on open rates, but consider going a little deeper and also compare click-through and click-to-open rates.

Click-through rates (CTR) determine how many emails resulted in a recipient clicking on your call-to-action links. For example, if you send 100 emails that gain 20 opens and 5 clicks, you’ll have a CTR of 5%.

Click-to-open rates (CTOR) compare the number of people that opened your email and clicked on the call-to-action — an indicator of how well your overall email performed. CTOR takes into account the number of people that also opened the email — so the CTOR for that same email would be 25% because a quarter of the people actually opened the email, then clicked through.

Is CTR better than CTOR? That depends on how you evaluate your campaign.

When analyzing your results, remember:

  • Open rate indicates the subject line’s performance.
  • CTR reflects an email’s total performance.
  • CTOR reflects the email content’s performance.

Step 4: Pick your winner

No test should last forever, and this also goes for A/B subject line testing. Your results will likely look different after one hour, 12 hours, and 24 hours. Plus, it’ll help you determine a true winner.

When measuring your results, analyze them within the overall context of your campaign. If your open rate was good but your click-through rate is low, did your subject line make a promise your email content didn’t deliver?

If your test is part of an active campaign, send the winning subject line to your full list, but today’s winner won’t necessarily be tomorrow’s victor. Great subject lines (and email marketing campaigns) require constant testing to stay strong.

Continue testing email subject lines

Even if your email platform doesn’t have built-in A/B subject line features, you can run a manual test and then use a spreadsheet to compare results.

Just remember:

  • It’s best to test one thing at a time.
  • Both subject line tests should be sent to the same number of people.
  • Both subject line tests should be sent at the same time.

Audiences’ tastes and engagement patterns shift, which is why you should never stop A/B testing. After all, one test doesn’t guarantee that your subscribers’ subject line tastes won’t change.

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