2015 Subject Line Analysis Report

White Paper

Following on from their wildly popular report in 2013, in this year’s edition Adestra has taken things to the next level. They’ve looked at more than 3 billion emails, and picked over 300 keywords across four industries:

  1. Retail & B2C
  2. Conferences & Events
  3. Media & Publishing
  4. B2B, Professional Services & Financial

Because your subject line is the key way to get your brand in front of your customers, your subject lines deserve lots of attention, and this report will be your guide to optimising them. You’ll learn what words cause higher open rates, and which ones cause more unsubscribes

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Welcome to Adestra’s 2015 Subject Line Report.

Following on from our wildly popular report in 2013, in this year’s edition we’re taking things to the next level.

In this report, you’ll find how more than 300 keywords across 4 industries affect open, click and unsubscribe rates.

This report was generated by looking at anonymized client data, covering:

  • more than 125 k individual email campaigns
  • more than 3 bn attempted email sends
  • more than 400 m opens
  • more than 55 m clicks

This huge amount of data gives you access to some of the most statistically robust information about the effect that individual keywords have on response metrics.

With this report, you’ll be able to determine how your subject lines compare to a huge, global average. It will give you unbeatable insight into whether or not your current word selection is positive or negative… and will give you a litany of ideas of what to test.

How we did it

(Warning – there’s some statistics in here)

We took 3 billion emails, and selected the most popular single words and two-word phrases.

We then split it by industry, and looked at the relative lift or decay of that word versus the industry sector average.

[Download PDF to see Image]

Email variance has a noticeable effect; however, with such a large sample size, it trends to a normal distribution.

While the average metric is interesting, it doesn’t tell the whole story.

To deliver you a measure of statistical reliability, we treated each keyword’s probability of success as a binomial variable. Then, we standardized each keyword and determined 95% two-tailed confidence intervals. The way to interpret this is simple:

“If you were to use this keyword in your subject line, 95% of the time the results will fall within the confidence interval bands.”

This method allows for a broad interpretation of the causal effects of individual words on response metrics.

But further, it gives you a solid idea about what works all the time, what works some of the time, and which words deliver unpredictable results.

The point to bear in mind is that any single word won’t always work for everyone all the time. However, these long run trends give you a good idea about good words, sentiments and propositions.

It’s not the end solution to your subject line questions, but it will point you in the right direction.

In the following pages you will find a huge amount of statistics and analysis. Bear in mind that we are not saying that any given word will deliver you 100% uplift on your open rates. This report is based upon global variance estimates.

With that said, the information is incredibly powerful. You’ll learn what words cause higher open rates, and which ones cause more unsubscribes.

You’ll be able to improve your subject lines like never before.

Why are subject lines so important?

It’s no surprise that subject lines matter. But exactly how much do they matter?

There are a few main contributing factors to an email being opened:

From Name: The from name can be tested to some extent. For example, using your brand versus a person’s name… beyond this, the options are limited.

Data Collection: A cleaner list will give you better results, but you can only control this to a certain extent. If you only sent out emails to people who have clicked in the last month, guess what? You’ll have an amazing open rate… but it won’t mean much, as you’ll be ignoring the vast majority of your customer list.

Day of Week and Time of Day: According to our analysis, sending emails out on Tuesday at 4pm statistically generates the best response. But, this is fleeting. If you only send out emails on Tuesday afternoon, soon everyone else will be too. And soon, Tuesday at 4pm will be the worst time to send out emails. Further, time of day is more reliant on externalities – that is, external variables over which you have little control – like, for example, the weather. Therefore it’s a difficult variable to test, and an even harder one to control.

Subject lines, conversely, offer you much more control and flexibility.

You can’t control the weather, and have limited power over the data you have access to. The from name options will be limited by your brand identity. But the subject line gives you unbeatable opportunities to test and learn about your audience.

Whether or not your email gets opened, people see your subject line.

What this means is that a subject line builds your brand in your customer’s inbox. Whether or not it gets opened, it creates a psychological association between your subject line and your brand. Over time, it creates a cognitive association between the language you use, and the perception your customers have about you.

Your subject line is your key way to get your brand in front of your customers.
Your subject lines deserve lots of attention, and this report will be your guide to optimizing them.


After brainstorming a few hundred words, we ran them all against the sample. Here are the top-10 and bottom-10 performing words across the billions of emails:

The first thing to note is both “Thanks” and “Thank you” appear in the top 10. These words are usually used in automated, transactional messages.

[Download PDF to see Graph]

But beware – mundane content will continue to drive poor or middling results. Nearly all of the worst words are things like “Journal,” “Whitepaper,” and “Report.” Your customers still buy benefits, not features, and the numbers go to prove this point.

[Download PDF to see Graph]

When customers perform an action on your site, they expect to receive some sort of instantaneous information.

Common logic dictates that you’ll get fantastic open rates – think about it yourself. When you purchase something online and get emailed a receipt, you nearly always open it!

And interestingly, excitement works! More exclamation points explaining more email benefits drive open rates and click rates.

However, individual words show varying results depending on context. In the following sections, we’ve split it into four key industry sectors to help you determine how your market responds to the popular keywords and phrases we’ve chosen.

Retail / B2C

In retail, time and time again it has been shown that email marketing is the number one driver of digital ROI.

Online retailers have larger lists than anyone else, and use email more frequently than others. The inbox competition is, however, more intense than ever, especially with the advent of things like Google Promotions Tab. The subject line has never been more important.

The first thing to consider is offers. Many sales are driven by specific percentage discounts, each of which have a substantial effect on both open and click rates, as follows:

[Download PDF to see Chart]

What’s interesting here is that when the discount gets larger, the open rate is less reliably improved. Why is this? One potential reason is that consumers don’t believe large discounts are real. Or, potentially, group buying sites have rendered large discounts as spammy. Think about it – how many emails do you get offering 50% discounts on something? If it’s too good to be true, it probably is.

But, for click rates, something else happens:

[Download PDF to see Chart]

Aside from 15%, all of the offers deliver higher click rates than the average. The logic here is simple – when users are interested in an offer, their propensity to click is naturally higher. This would indicate that customers convert at a higher level.

And better yet – offers like these can actually reduce unsubscribe rates:

[Download PDF to see Chart]

While the results are quite variable, the reason for this phenomenon is because people don’t want to miss out. Perhaps this offer isn’t the one for them, but the next one may be.

Retailers try to offer personalized experiences to customers across their online channels. However, this proves to be risky, as indicated below:

[Download PDF to see Chart]

Personalizing is certainly something worth testing, but the point is to sell from your email. If personalization adds value to the sales funnel, go for it. But if not, focus on your value proposition instead.

Conferences / Events

In the conference and events industries, the main problem remains a simple one: differentiation and homogeneity.

For example, the following chart shows the common features at an event:

[Download PDF to see Chart]

All of these are key features in many events, and yet they don’t deliver strong open rates. The reason for this is because they focus on the feature, and not the benefit.

There is an even more pronounced result when looking at clicks:

[Download PDF to see Chart]

This response decay is largely due to a lack of creativity or propensity to experiment with other tests. The problem comes down to event workflows. Generally, the prime registration period comes in the weeks leading up to an event. At this point, marketers are overwhelmed with a million and one tasks. The last thing they want to think about is a subject line! However, this is done at their own peril – considering email is one of the strongest visitor drivers out there, optimizing it is your #1 route to increased footfall.

But, what to bear in mind is that an event invitation is inciting an online transaction. It’s very important to use strong calls to action, even in the subject line, if you want your customers to respond to your invitations.

Here are some of the more common call to action words we tested:

[Download PDF to see Chart]

Clearly, some terms (i.e. “early bird”) are over-used and don’t deliver great value. But take, for example, “Buy.” It’s a strong call to action, often associated with retail transactions, but very effective for event invitations as well.

And the same holds true for clicks:

[Download PDF to see Chart]

Email’s purpose in the event sector is to get people to visit your registration page, and to have them turn up on the day. Email is the perfect channel to make this happen, so long as it’s done in such a way that recipients feel interested and valued.

Media / Publishing

The media and publishing industries continue to undergo industry challenges, and yet through all of this, email has proven to be a reliable channel to generate subscriptions and page views. However, different purposes are served with differing success by email.

First, editorial content.

While not usually a revenue generator in itself, the publishing world is still driven by editors. It is their industry insights that create traffic and ultimately long-lasting customers. The editors become the voices of a given niche, and this value is strongly evidenced in words with positive outcomes, such as:

[Download PDF to see Chart]

It’s no surprise that these are eye-catching words, but the effect on click rates should be looked at as well.

The majority of publishers send email newsletters to:

  1. increase subscriptions and
  2. increase page views.

So opens are great, but ultimately they need more page views.

[Download PDF to see Chart]

Strong editorial content, when used as a hero topic in the subject line, drives huge response across all metrics.

Often marketers and editors operate in separate ways; however, these statistics indicate that a more joined-up approach between the two departments will result in a better overall email product.

The second main purpose of email for publishers is to sell subscriptions to their titles, be they online or offline. However, this is easier said than done, and the important thing is to consider what words will work with your audience. Some examples are in the following table:

[Download PDF to see Table]

The word choice makes a huge difference in the sale of subscription products.

However, this is not to say that discounts and % offs never work – they do in the right context, as evidenced by the strong variability. This is even more stark when looking at clicks:

The point not to stop using offers in your emails. They work lots of the time! But, they don’t work all the time. It’s important to change things on an email-to-email basis.

[Download PDF to see Chart]

B2B / Professional Services / Financial

In any given B2B niche, there is any number of direct competitors or indirect substitutes for any given product.

The challenge in this sector is to stand out from the competition in an intelligent way. This can be achieved in numerous ways – popularized, for example, by content marketing and native advertising.

[Download PDF to see Chart]

However, the various methods available to marketers deliver mixed results, as evidenced by the following word choices:

The idea to bear in mind is that B2B product offerings are often complex, and the sales cycle is much longer than other industries.

This means that the consistent provision of news items is pivotal to long-term success. However, if the content is of marginal quality, or perhaps not targeted correctly, the results will be a disengaged user base, as shown by the click decay:

[Download PDF to see Chart]

When unsubscribes happen in B2C industries, it’s not a huge deal because the average customer value is relatively low. However, when unsubscribes happen in the B2B world it’s much more important – especially for high-value products with a convoluted sales process. Following an email strategy that reduces unsubscribes is solid. In the following chart are a number of words that either substantially increase or decrease unsubscribe rates:

[Download PDF to see Chart]

If a key strategy for your business is to maintain long-lasting connections with prospects to ensure you are in the right place at the right time, it is very important to avoid unsubscribes.

[Download PDF to See 10 Comprehensive Tables Containing Statistics on Open Rates, Click Rates, Unsubscribe Rates, Open Rate Variances and Unsubscribe Variances Across B2B, B2C Sectors and More]

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