Subject-Line Optimization: Don't Test Your Way To Mediocrity

The good news: Many marketers test their subject lines. According to the Direct Marketing Association (U.K.), 80% of marketers use some form of subject line testing.

The bad news: Many of the most commonly used subject-line tactics actually decrease results. In some cases, marketers are testing suboptimal approaches.

Subject lines, in addition to being the factor that most directly affect read rates, are among the easiest elements of a campaign to test. The results are clear, quantifiable, and quick; you don’t even need to wait until the campaign is fully deployed. Assuming your campaigns are reaching your audience’s inbox, nothing else you can test offers more upside than subject-line optimization.

We recently reviewed the read rates for millions of subject lines across millions of subscribers in our consumer data stream. When we normalized for the activity level of each email recipient, we found that many commonly used approaches that don’t seem to work.

Examples include:



Messages with price-focused subject lines had lower read rates than those whose subject lines took different approaches. Subject lines referring to dollar values were read 0.8% less frequently than others; subject lines referring to percentages were read 1.0% less frequently.

In general, deals didn’t intrigue people as much as urgency. Subject lines referring to “expiring” were read 1.6% more often than others; those referring to “last chance” were read 1.1% more often.

“Discount” themes – a common approach – were relatively weak. Subject lines touting “clearance” coincided with read rates that were 3.8% lower than similar messages with different subject lines; “save” subject lines were 0.6% lower.

Bottom line: Even a disciplined testing program, if it started with a bake-off between “price” and “discount” subject lines, might be merely working toward the least ineffective approach – testing its way to mediocrity, not greatness. Beginning a testing program with other senders’ proven winners (for your subscriber base) is likely to deliver better results faster.

I always say your results may vary. Deep discounts may work better than deadlines for your readers. Even clickbait subject lines (e.g., “The shocking secret chiropractors don’t want you to know”) might be the best way to get people to read your messages -- although you’d be an outlier.

You won’t begin to know until you test. Your tests will be more efficient if you start with subject lines that work for your subscribers “in the wild.”

3 comments about "Subject-Line Optimization: Don't Test Your Way To Mediocrity".
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  1. Skip Fidura from dotmailer, May 6, 2015 at 10:56 a.m.

    George -

    Interesting stats. Was the data for the analysis limited to the UK as well? I only ask as it does not tie up with another UK DMA Email Council research report which shows the number one driver for consumers to sign up for brand emails is deals and offers.

  2. George Bilbrey from, May 6, 2015 at 11:30 a.m.

    We did not limit our analysis to the UK.  Results may vary by country.  However, it wouldn't surprise me if what someone says on a survey and how they act  day-to-day are different.

  3. Cynthia Edwards from Razorfish, May 15, 2015 at 4:02 p.m.

    I agree with the thrust of the article. Another pitfall is to assume that because a certain element of a subject line - such as personalization or a cute icon - showed higher results in a test, that that element should appear in EVERY subject line going forward. What was once new and surprising will become old and stale quickly.

    My strategy is to follow principles learned in testing AND continue to keep subject lines fresh. Change is good! 

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