Reality: In the early days of email, the open rate was a valued metric because it captured who opened and, by inference, who then viewed or read the email. Direct marketers drooled over this new metric because it gave insight into the first stage of customer engagement and was not possible with direct mail.

Today, the preview pane and image blocking have turned the open metric into a tired, inaccurate and irrelevant metric that no longer measures what it was originally intended to.

• It's inaccurate: Blocked images will prevent an open from being recorded when the tracking image fails to load on the hosting server. Text opens also cannot be tracked, and emails with images loaded and read in the preview pane are counted as opens even though the email is not actually opened. While no hard numbers exist, I estimate that open rates typically are under-reported by 10-30 percentage points.
• It's irrelevant: The open rate is a process metric, which doesn't measure actual campaign performance against goals such as ROI, conversions or revenue. And its inaccuracy simply makes it unreliable as a process metric.
• It's unsophisticated: The open rate doesn't distinguish between those who read the whole message and those who open and then trash it; or between viewing the email in the preview pane versus fully-opened.

In recent years, as more ISPs and email clients blocked images and offered preview panes by default, open rates began to fall. And now, more email users are reading messages on mobile devices, most of which don't render images.

Recent email studies estimate that both consumers and business email users view from a quarter to a half of their emails in a preview pane, and 50-60 percent block images by default.

Do marketers really want to base performance reports and marketing decisions on such a flawed and inconsistent metric?

The eec Measurement Accuracy Roundtable, of which I am co-chair, is working to redefine, rename and standardize the open rate. Unless some miracle technology emerges, though, it will remain a non-strategic metric.

So, use the open rate if you understand its limits and can derive lessons to improve your email program. Better yet, find the key output metrics that will enable you to gain a larger share of the marketing budget.