Previously, we found that ThriveHive customers' email campaigns achieve open rates significantly higher than the industry standard. This difference, however, varies by industry. For example, Computers & Electronics and Entertainment & Events businesses perform below industry standard, while Health & Fitness and Professional Services perform far above the industry standard.

  So the question remains: Why are email open rates higher for certain  industries?

  Of course, part of the answer to this question is intrinsic to industry. Industry-level differences were pointed out in the previous blog post and have been found in other settings . But to really dig deep and understand some of the possible root causes of these differences, ThriveHive might need some kind of Data Scientist.  Luckily, they have one.

  First, I wanted to understand what's driving open rate. When customers receive an email, there are three main things they look at: the sender, the subject line, and the first few words of the email. In this and the next blog post, I will focus on subject line.

Subject Line Length

  One characteristic of subject lines that has received research attention is the character length . Email clients like Gmail typically display 50-70 characters of a subject line. Customers accessing their email on mobile devices might be limited to closer to 30 characters. If a campaign's subject line is getting cut off, businesses can have trouble communicating the message or content to the recipients, which may be related to low open rates.  

  However, there is a debate as to whether there is an association between subject line length and open rates.

   Among ThriveHive customers' campaigns, subject lines with six or more words (or more than 30 characters) had significantly lower open rates. There was also an appreciable drop in open rates between subject lines 70-79 characters and 80-89. 

Open rate by subject line length

 It appeared very short subject lines (0-9 characters) had very high open rates, though that result showed a lot of variability. Looking more closely at single-word subject lines showed that a small subset of businesses had a tendency to use words like "goals" and "party" or a slew of words related to food groups (e.g. "dairy").  

  This result suggests that there may be some industry-level effects of subject line length, as the single-word subject lines seemed particularly closely associated with specific industries. To test this industry-level effect, I used a Generalized Linear Model of open rates on several email characteristics including character length and industry.

Subject Line Length + Industry

  The model showed that, on average, longer subject lines had lower open rates. As suggested by the previous blog post, subject lines longer than 29 characters had significantly lower open rates.  

   Interestingly, this effect appears to be happening more commonly in the Health & Fitness industry. Within the Professional Services, Entertainment & Events and Computers & Electronics, subject line length appeared to matter less.  

Industry-subject line length relationship to open rate 

  In the above graph, you can see that Health & Fitness emails with subject lines longer than 29 characters have 15% lower open rates than emails with shorter subject lines. Among other industries, the difference is smaller, about 5%.

 This could mean a couple of things:

  1. Businesses in the Health and Fitness industry communicate effectively to their customers with short subject lines.  Hubspot's blog has an example of a restaurant delivery platform that, by having established punctuation shorthands for in-app notifications, can send an effective subject line that's just one character: "!".
  2. Subject line length doesn't matter, content does. There may be something intrinsically different about Computers & Electronics companies that makes length less of a factor in their subject lines. There are definitely differences in open rate generally, so there are also likely to be differences in response to features like subject lines.

Summing up

Since I'm a scientist, I'm inclined to give the scientist's conclusion here: More research is needed. Evidence suggests that emails with longer subject lines have lower open rates, which matches existing research.  Particularly, subject lines shorter than 30 characters seem to have particularly high open rates. However, this is a relationship that differs by industry, so it's definitely worth testing with your audience before going full-steam ahead with one-word subject lines. It's important to test new strategies if you're unsatisfied with your email campaigns' performance. Remember, there's no silver bullet, just a lot of trial and error.