The email open rate
tells you how many of your recipients actually opened your email. Open rates are generally determined by a tiny one pixel by one pixel transparent gif somewhere in the layout of the email. When the email is opened, a request is received by your server to provide that image, just as if it was a product photo or a background pattern. Once you have your total number of transparent image requests tallied, it is expressed as emails opened / emails sent: that is your Open Rate. However, this may not be the most accurate representation of how many of your emails are actually opened!
The Mobile Revolution Has Devalued Open Rate Calculations
Many email marketers continue to rely strongly on open rates even though recent developments in email clients and the explosion of mobile web enabled devices has severely handicapped the accuracy of open rates. Because many of these email readers have images turned off by default, none of the images included in your email become visible to these recipients. Unfortunately, this includes that critical 1 x 1 pixel transparent gif that communicates the "open" to your server. Therefore if you are sending emails to 100,000 users, with half reading your emails on desktop or laptop computers through email clients that allow images, and the other half reading them on various devices that prohibit images, then your open rate calculation is severely underestimating the actual number of emails read… by fully one half!
Extrapolated Open Rate May Be a Closer Approximation
Although it has not yet received wide industry adoption, an Extrapolated Open Rate calculation may be derived through an analysis of the browser types that your subscribers are using and correlating that to your open rate. Even though it is effectively impossible to make an accurate determination as to exactly which users will have images on vs. off, it's a fairly safe assumption that most mobile - and a reasonable percentage of large provider services such as Gmail and Hotmail - will be set to off. In this case a sample Extrapolated Open Rate calculation could be:
100,000 emails sent
20,000 emails reported by transparent gif as opened for an Open Rate of 20%
Conventional images on email clients: 60%
Assumed images off email clients: 40%
Therefore 60,000 emails resulted in 20,000 opens for a rate of 33.3%, a rate which can be extrapolated as closer to the actual open rate for the entire 100,000 emails sent.
Massive (Negative) Effect on Click-Through Rate
Since open rate is the basis for that all important click-through rate
(CTR) calculation, the differences between a CTR determined through conventional open vs. an extrapolated open can be enormous. Let's assume that in your conventional open calculation you show your CTR as 5%. However, that is based on a total open of 20%: Out of 20,000 opened emails, 5% or 1,000 were unique click-throughs. The numbers fall considerably if you're using the extrapolated open figure of 33%. In this case out of 33,300 opened emails, 5% is no longer 1,000, but 667 or 3.33%! Of course there have always been some recipients who read emails with images off, so to panic and think that your CTR has plummeted from 5% to 3.3% overnight is an overreaction. Consider the extrapolated CTR to be a more accurate estimate, not a reflection of your campaign's sudden failure.
At the current state of technological development, there is no real way to determine that the reason for the transparent gif not being requested to your server was because the email was unread, deleted outright or displayed as images off. Perhaps one day in the distant future some email Einstein will figure out a way to have images off emails call home and duly inform the sending server. Until that time, the best we can do is craft email newsletters
that users want to read and will enjoy keeping images displayed for.