How will Google image caching affect my analytics?
If you are using individual subscriber tracking (through a Merge Tag), Google’s image caching should be a minimal impact to your Analytics tracking. You’ll see fewer Total Opens, because after downloading the Pixel Tracker Image the first time in that email client, that recipient will download the image from Google’s proxy server each subsequent time.
For example, if the subscriber opens the email three times in the Gmail App on their iPhone and 2 times in the Gmail web client on their desktop, you’ll see 2 opens in your analytics report (one open from the Gmail app and one from the Gmail web client). You’ll probably see more Unique Opens than you did before Google image caching, as every Gmail user who opens it will automatically download images.
If you’re not using a Merge tag to track individual-level data, you’ll see significantly fewer Unique Opens in your reporting for Gmail. The image will be downloaded the first time and cached, then subsequent image downloads will be from Google’s proxy server. This could have a much larger impact on your analytics and show erroneous reporting for your Total vs Unique Opens. For this reason, we recommend that individual subscriber tracking be used for all analytics campaigns.
Google image caching only applies to recipients who access their Gmail through the Gmail web (browser) interface, or a Gmail Mobile App such as what you may find on your Android or iPhone device. Recipients who access their Gmail through a desktop client (such as Thunderbird), will download images as they normally would in that client and report accordingly.