Ways to Test Subject Line Performance

If you have spent any amount of time crafting sales emails, you know the importance of a good subject line to its overall performance. You also know predicting which subject lines will set your bottom line on fire and which will leave your readers cold is more like fortune-telling than weather forecasting. Here are a few tricks you can use to find out which subject lines are going to do well and which ones aren’t.

Get a Head Start

One of the quickest ways to test potential subject lines against each other without exhausting your email list is to use pretested subject lines from online ad campaigns such as Google AdWords campaigns.

Create a Level Playing Field

If you’re going to get an accurate idea of which subject line will perform best, you need to create a level playing field. That means everything else must be equal.

  • Take a random sample from your larger list to test, without segmenting. Don’t send one subject line to your best customers and the other to tire kickers. You might want to test different approaches to different list segments later on, but for now, you want each subject to have the same chance of conversion.
  • Keep everything else in the email the same, and point the links to the same page. Include a #ref (e.g. http://www.yourdomain.com/yourpage.html#ref1) for each subject to make tracking simpler.
  • Try to make the page you’re directing traffic to equally relevant to each subject line. If the page is more closely aligned to the subject of one email than the other, that could affect conversions. In this case you could create two pages, again identical except for the heading, which would reflect the subject line of the referring email.
Test Performance End to End

It’s tempting to either only measure open rates and clicks to your Web site, then transfer your attention to on site optimization, or to concentrate solely on conversion rates. Either approach could leave you missing out on valuable information, however. Measure:

  • The open rate as a percentage of deliverable emails, to see which subject line is most immediately effective.
  • Click through rates to your Web site, to see which subject line supports the message of the email best.
  • Drop-out rates at each stage of conversion to identify any areas of friction. If these are consistent across subject lines, onsite changes are indicated. If one subject carries a greater percentage through a particular barrier than the other, though, you have uncovered important insights into your customers’ behaviors.
  • Consider overall conversion rates. It may well be that the subject line with the highest open rate is not the one with the most conversions.
Tweak and Retest

If your testing revealed a clear winner across all stages, you will probably want to send that out to your full list with no further changes. If your highest open rate converted poorly, however, you might want to run a second test, perhaps with the higher converting subject as a heading within the email body. If neither subject line converted satisfactorily, you could try a third option. In any case, when running the second test, the highest performing option from the first test should be rerun alongside as a “control.” This reduces the effects of timing differences on the results.

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