Six case studies and an infographic on how to write effective email subject lines

There are many factors that influence whether or not people bother to open your marketing emails or not.

Email frequency, brand affinity and time of day are all influential, as is the subject line that marketers opt for.

The precise wording will vary depending on the brand identity and the product offer, but the basic aim is to grab the reader's attention and make them interested enough to clickthrough and find out more.

To help get to the bottom of what makes up the best subject line, I've rounded up a few case studies and infographics that try to shed some light on the topic.

And for more information on email marketing, check out our Email Marketing Census and our blog posts looking at how fashion retailers deal with the sign up and opt-out processes...

Three phrases to avoid: Help, % off, and Reminder

Email provider MailChimp identified a number of keywords and offers to avoid like the plague after analysing the open rates for more than 200 million emails across a range of different campaigns.

It found that the phrases 'help', '% off' and 'reminder' all have a negative impact on open rates, as do full caps and exclamation marks.

Often these phrases are associated with needy or begging emails, or an offer that's too good to be true.

On the plus side it found that:

• Including a person's name doesn't impact the open rate but localised content, such as including a city name, does help.
• Ideally subject lines should be kept to 50 characters or fewer. The exception was for highly targeted audiences, where the reader apparently appreciated the additional information in the subject line.
• Subject lines framed as questions can often perform better.

Clarity vs. Creativity

Communications blog AWeber ran a test that compared creative email subject lines against those written in a clear, straightforward manner.

Subject lines written creatively included:

• AWeber's AWesome Anthony A.
• Getting Earth-Friendly Beyond Email.
• Threadless' Frequency Alert: Hot or Not?

While subject lines written with clarity included:

• Grow Your Email List 99% Faster: How One Site Did It.
• 43 Free Animated GIFs For Your Email Campaign.
• Email Timing: A Look At 6 Marketers.

The test was run across 20 subjects line and sent to a list of over 45,000 subscribers. The results aren't particularly surprising as the subject lines it describes as 'creative' could also be labelled 'vague' or 'uninteresting', but across each channel the clear, descriptive subject lines far outperformed the creative versions:

Descriptive vs. Personal

Online petition site Credo Action tested the efficacy of different types of email subject lines in order to increase the number of people signing up to its campaigns.

In this case the experiment compared the success rate of descriptive headlines (e.g. "Tell Obama: Justify your indiscriminate spying on Americans") against more personal subject lines (e.g. "I just signed this – will you?").

In all cases the personal subject lines convinced at least 30% more people to sign online petitions and in one case achieved a 90% increase.

Another MailChimp report

Getting to the bottom of the subject line conundrum can't be achieved in a single study so MailChimp ran another survey, this time analysing more than 40 million marketing emails.

It observed that subject lines you might categorise as descriptive, straightforward, or even dry, tend to achieve the highest open rates. These were things such as '(company name) Jan/Feb Newsletter', '(company name) May News Bulletin)' and 'We're throwing a party'.

In contrast, those that achieved the lowest open rates tended to be more salesy and more obvious in their attempts to grab the reader's attention.

This includes examples such as 'Valentines – Shop Early & Save 10%', 'It's still summer in Tahoe!' and '(company name) holiday sales event'.

The golden nugget of advice that MailChimp pulls from the research is that subject lines should simply "describe the subject of your email."

If your email is a newsletter, put the name and issue of the newsletter in your subject line. Because that's what's inside. If your email is a special promotion, say so in the subject line. Either way, just don't write your subject lines like advertisements.

Subject line tests lead to 19% boost in conversions

Email strategy consultant Jeanne Jennings blogged the results from a subject line test she ran for one of her clients.

The business involved wanted to increase site traffic from its email newsletter in order to encourage its users to continue subscribing to the service.

Jennings compared the standard control headline, 'Subscriber newsletter', against subject lines that included a key theme in the first 25 characters.

In the final results, the test subject line out performed the control in all four click-through metrics.

The most important figure to look at is the click to open rate (CTOR) which shows that the test subject line did a better job of getting subscribers to click on links within the emails.

152 killer keywords

To find the ultimate subject line Adestra tested a random sample of 95,000 global, English-language campaigns over 12 months (for a total of 2.2 billion emails), and isolated 287 popular 'trigger words'.

Then, split by sector, it looked at the correlation between the word's inclusion in the subject line and its variance above or below the average results for key email metrics (open rate, CTR, CTOR, and unsubscribe).

Some of the key findings were:

• Specific trigger words have a drastic effect on the response rates of offers. For example, 'Sale' delivers +23.2% opens, but 'Save' only +3.4%.
• However, in terms of clickthroughs, they give +60.7% and -25.2% respectively.
• Content marketing headlines doesn't always work. For example, 'Report' (-23.7% opens, -54.8% CTR) and 'Webinar' (-16.6%, -70.7%.) failed to live up to expectations.
• Conversely if the content is good, people will consume it. So 'News' (+34.8%, +47.7%), 'Bulletin' (+15.8%, +12.7%) and 'Video' (+18.5%, +64.8%) work well.

And finally...

Here's an infographic from Litmus on how to write the perfect email subject line. Enjoy!