As email marketing has matured, measuring the channel’s performance has evolved substantially. Yet all too often, marketers rely on rudimentary campaign metrics from the batch-and-blast days of the past to judge its performance.

Instead of thinking exclusively about how well a particular campaign offer or creative performed, marketers should additionally look to longitudinal metrics – measurements of subscriber engagement over time to determine whether or not an email programme is successful.

One of email’s greatest strengths is its measurability. Therefore, marketers regularly evaluate campaigns against standard response metrics such as delivery rates, open rates, click rates and conversions. These are all excellent criteria for how well a particular offer or message resonated with the target audience. Further, those who are willing to test multiple combinations of copy and presentation (from lines, subject lines, content, creative, layout, etc.) can achieve highly optimised rates of response at a campaign level.

The next level of reporting, however, moves beyond simple campaign metrics and looks at subscriber engagement over time. This type of historical response analysis provides insights into which customers are engaged with the programme and which are not.

Additionally, by adding a dimension of time, marketers can determine which subscribers have increased engagement with the programme and which have lapsed into inactivity. This customer-centric view of response activity provides a much richer understanding of how well a programme is performing at driving lasting customer engagement.

For example, below are some industry-specific results from Epsilon’s quarterly Email Trends and Benchmarks that look at longitudinal customer engagement, revealing that certain industries are better than others at maintaining on-going subscriber interactivity . The first column shows that – on average – 18% of a typical subscriber file – both new and old subscribers to an email programme - has engaged with an email (opened or clicked) during the past three months, significantly higher for consumer retailers, and lowest for healthcare clients.

The more-dramatic results in the second column show increased engagement at the beginning of a programme lifecycle, with 34% of new customers engaged with the programme – nearly twice as much as the “average” subscriber on file. New subscribers tend to be more engaged with an email programme during this “honeymoon” period (first few months on file), underscoring the importance of a good email “welcome” series that captures initial interest and establishes a valuable relationship for ongoing communications.

By blending in purchase activity we can begin to tie email channel engagement to actual revenue. Whether email engagement and purchase activity are cause-and-effect or simply correlated, we can at least prove (or disprove in some cases) that we are reaching and engaging our most valuable customers.

Below is an example of how email programme engagement overlaid with real purchase activity can help email marketers tie programme engagement to top line revenue. In this example, consumers who clicked on emails have the highest median purchase value. Those who opened the email also spent significantly more than inactive recipients, or those who neither clicked nor opened an email.

Epsilon has also found that those who actively engage with the email generally make more frequent, - higher value purchases than those who are unengaged with the programme. In addition, we see new-to-file email addresses perform well above average during their first three months in the programme, when they tend to be most engaged. Adding purchase data to the mix not only helps marketers to further segment the file by purchase type and activity, but also helps connect email engagement with revenue to demonstrate true programme productivity.

Longitudinal engagement is a powerful way to analyse customer engagement over time. It provides email marketers with an immediate segmentation opportunity. They know which subscribers are active and engaged so they can target them for continued loyalty. They can identify dormant/lapsed responders and focus on reengagement. Lastly, email marketers can segment the disengaged customers and target them with win back and reengagement messages. Not all customers will respond to the same messaging so a deep dive into your email programme performance, will allow you to understand the specific needs for each segment and launch appropriate programmes that focus on the opportunities for engagement.