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While there’s no silver bullet when it comes to creating a sticky subject line, it doesn’t hurt to turn to the data scientists for a bit of advice. Thanks to the painstakingly-detailed “Subject Line Analysis Report” from Adestra, light has been thrown on some of the most commonly-used words and phrases used, alongside what influence they have on opens, clicks and unsubscribes.

Assessing the impact of words like “free”, “limited time” or “register” in subject lines is undoubtedly a tricky proposition, given that they are just one of a squillion variables that contribute to how an individual campaign performs. Given this trickiness, we couldn’t help but have our curiosity piqued by the extensive subject line analysis presented in Adestra’s report.

In this post, we’ve picked out some of the most interesting findings. Keeping in mind that your mileage may vary from what’s presented in the report, let’s look at some of the best and worst performing keywords.

“Free Delivery”… But with a catch

As they say frequently in some parts of the world, “freedom isn’t free.” This applies loosely to subject lines, too – according to Adestra’s report, featuring the word “free” in subject lines results in a -3% drop in opens in comparison to the average amongst all email campaigns surveyed. However, the phrase, “free delivery” provides a 50% uplift… But also results in an 82.4% lift in unsubscribes, too. A similar trend can be found in subject lines containing “voucher” (20% lift in opens, 60.6% more unsubscribes) and “sale” (23.2% lift in opens, 31.6% more unsubscribes) – my personal assumption here is that subscribers are initially attracted to emails that appear to be containing a offer, but may be more likely to react by unsubscribing if they find their assumptions or desires to be left unfulfilled. One to discuss in the comments, no doubt.

On the other end of the spectrum, “report” is a real downer, resulting in a 23.7% decrease in opens). Similarly, “learn” also apparently sounds like a lot of hard work for many readers, averaging a 35.5% drop. While it might be tempting to look at the positively better unsubscribe rates for campaigns using these words, it’s worth noting that if a subscriber doesn’t bother opening a campaign, they’re not going to give themselves the chance to unsubscribe, either (assuming this wasn’t taken into account in the numbers).

When to send: Daily, weekly or monthly?

Another interesting finding is that subject lines containing the words, “daily”, “weekly” and “monthly” all perform differently – most likely, because this reflects their actual send frequency. In Adestra’s words:

There is also a strong logic in creating an expectation to receive scheduled messages. Communications that are sent out “Daily” and “Weekly” perform strongly. This is because your customers begin to expect your emails on a frequent basis, and get into the habit of reading them. At the other end of the spectrum, “Monthly” is probably too infrequent, and you’re losing the top-of-mind position for which you are striving.

While this isn’t directly correlated to language use in subject lines, it’s an interesting observation… Especially to us, as we send a monthly newsletter – which you should subscribe to!

Of course, the small print…

Adestra’s research is based on results from 90,000 email campaigns, each which have been sent to 5,000 or more subscribers – with impact being measured as the difference between the average open rate of emails with a subject line containing, say, the word “sale” and the industry average open rate. Same goes for their figures on clicks and unsubscribes.

Something to note is that variation between open rates may not amount to as significant a change as may initially seem – for example, if the industry average open rate is 20%, a 12% lift on this will result in an actual open rate of 22.4%. Of course, if you send to a large list, a 2-3% lift in open rates may make a big difference to your overall results.

Many thanks to Adestra for this painstaking work – you can download their report for full results (requires signup). Now, over to you – do you have a word, or phrase that works particularly well in your subject lines? Have you run an interesting A/B test on a recent campaign? Let’s share notes in the comments below.

This blog provides general information and discussion about email marketing and related subjects. The content provided in this blog ("Content”), should not be construed as and is not intended to constitute financial, legal or tax advice. You should seek the advice of professionals prior to acting upon any information contained in the Content. All Content is provided strictly “as is” and we make no warranty or representation of any kind regarding the Content.
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