A/B test an email subject line

Send two different subject lines to a portion of contacts and automatically send the winning subject line to the rest

Review your plan: This feature is not available in all Constant Contact plans. If you don’t have full access, you may need to upgrade your plan.

The easiest way to improve your open rate is with a great subject line; it's the first thing your contacts see when you send an email. Comparing two subject lines with an A/B test (a.k.a split test) ensures you're always sending the most effective subject line out to your audience.

• How subject line A/B testing works
• Enable subject line A/B testing

How Subject Line A/B Testing works

A/B testing is a way of comparing how version A of something, like your email subject line, performs compared to version B, within an audience, like your contact list:

1. An email with subject line A is sent to up to 50% of your selected contacts, while a second email with subject line B is sent to an equal portion.
2. After a specified amount of time, the subject line with the most opens is declared the winner.
3. The winning subject line is automatically sent to the remaining contacts who weren't part of the A/B test population.

A/B testing is a powerful tool that helps take the guesswork out of what your contacts find engaging!

The data shows: To achieve statistical significance, the recommended audience size for each subject line is at least 1,000 contacts. Feel free to send to fewer contacts, but keep in mind that the smaller the list size, the less certain we are that the higher open rate was due to the subject line.

Enable subject line A/B testing

1. When you're ready to send your email, click Continue.
   Note: If you edit the email header from the top of the email editor, that subject line will be added automatically as the "A" subject line on the schedule page, where you'll fill in the rest of the A/B test information.
   
   
    
2. Select the email lists you want to send to. The more contacts on your lists, the more statistically significant your results will be; a combination of lists that total at least 1,000 contacts is a great start.
   Note: Additional features, like filtering a list by tags or sending to a segment, may prevent you from achieving statistically significant results because they can reduce the number of contacts you're sending to. If your list is on the smaller side, we recommend saving those tools for another email that's independent of your A/B test.
   
   
    
3. In the Campaign info section, click the A/B Test toggle to enable the feature.
4. Enter the two subject lines you want to test in the A and B subject line fields.
   Note: The subject line personalization option is disabled when you enable A/B testing on the schedule page. However, if you create two personalized subject lines and copy them before enabling A/B testing, you can paste them into the A and B subject line fields. 
5. Use the slide bar to select a percentage of your contacts to send the test to. The rest of your contacts receive the winning email after the test is complete.
6. Select a time period to wait before declaring a winner to give the contacts in your test population time to open your email. 
   
   
    
7. When you're ready to start your A/B test, use the toggle to choose either:

• Send Now - To immediately send your A/B test to your selected contacts. Then click the Send Now button.
• Schedule for Later - To select a future date and time to send your A/B test to your selected contacts. Then click the Schedule button.
  
  

Once your chosen time period passes, an email with the winning subject line is sent to the remainder of your contacts who weren't part of the test population. The winning subject line is not re-sent to anyone who received the losing subject line as part of the A/B test. You can check your reporting to see your winner and use the results to improve the subject lines of future emails.

Questions?