One word

Subject lines are important, right?

Just switching up the subject line can often swing open rates by a pretty wide margin.

Thinking about subject lines in terms of open rates is a trap though.

Here's why:

I recently ran a simple test on an email list of around 80,000 subscribers.

I was promoting a free webinar that offered to show subscribers how to learn the Python programming language and start landing freelance projects with their new skill.

Right now, Python is one of the hottest programming languages among developers—it's a lot of fun to use, and it's the language of choice for a lot of exciting niches like AI and computer vision.

So I decided to try an A/B test with two different subject lines, each emphasizing a different aspect of the offer (emphasis added):

  1. How to become a freelance developer (free class)

  2. How to become a paid Python developer (free class)

I was curious whether switching out the focus like this would have any impact on opens, clicks, etc.

The difference ended up being pretty small.

The “Python” subject line narrowly edged out the “freelance” one—it actually got fewer opens but more clicks.

Based on that I was thinking that the Python angle was a slightly stronger appeal.

Then I did some digging into WHO opened each email, and I found an important difference:

While the subscribers who responded to the Python subject line tend to be more “engaged” with the emails—they open and click more—the subscribers who responded to the Freelance subject line have spent a lot more money in the past.

About 15% more on average.

On this particular campaign that gap could mean several thousand dollars of revenue.

The takeaway is:

Two subject lines could have roughly equal open rates but produce very different results, because of WHO is doing the opening.

And even swapping out one word can be enough to dramatically change the audience for a given email.