It can be difficult to measure small changes in delivery because there are seasonal trends in email. Heck, there are hourly trends in email. As you might suspect, Gmail opens do follow these hourly, weekly, and seasonal trends, but on the whole they remain fairly steady. With that settled, I trimmed my data to a lean 6 weeks around the introduction of tabs. That’s about 1.5 billion emails, which is plenty of records for a good analysis.

via How Gmail’s New Inbox Is Affecting Open Rates | MailChimp Email Marketing Blog.