Industry research and undoubtedly your own observations indicate that Web users frequently delete cookies.
Since Google Analytics depends on cookies for user, session, and source tracking, their deletion has the following significant impacts:
- New visitors skew higher, and returning visitors skew lower.
- Unique visitors skew higher.
- In the Frequency and Recency report, Count of Visits (per visitor) and Days Since Last Visit both skew lower.
- More direct traffic is reported. If you click through to mysite.com from a specific source such as a Google organic search result on Monday and then enter mysite.com directly into your browser address bar on Wednesday, your Wednesday visit still counts as organic – unless you delete your cookies on Tuesday, in which case the source and medium are lost and your Wednesday visit counts as direct.
These same effects arise, of course, when a visitor returns using a different browser, since cookies aren’t shared among different browsers on the same computer and certainly not among different computers altogether.
There’s no specific formula that can offset the loss of return visits, so just keep the cookie deletion (and multiple-browser) factors in mind as you review you Google Analytics reports. There is always a greater percentage of return visits than Google Analytics can report.