For analysts like you and me who depend on the multi-session perspective that Google Analytics provides, cookies are a sweet treat indeed. With just a few lines of plain text stored in the browser, cookies allow Google Analytics not only to report on returning visitors, but also to map conversion attribution through multiple traffic channels and even through offsite marketing campaigns.
While cookies have greatly helped to overcome the problems that dynamic shared IP addresses posed to tracking across sessions in the days of log file analysis, they’re not a cross-session cure-all. Cookies are not shared among browsers or devices, and by many estimates, 30% of Web users delete cookies on at least a monthly basis. When you’re interpreting Google Analytics reports, it’s important to remember the dependency on cookies as well as the broad implications of cookie deletion and multi-browser/device usage.
The _ga Cookie
The first task Google Analytics performs on a Web page is to write a new _ga cookie to identify the user as unique. If the _ga cookie already exists in that browser for the given site, the Google Analytics instead refreshes the expiration for a full two years.
Your browser stores a _ga cookie for each Google Analytics-tracked website that you visit.
Cookie Deletion, Multiple Browsers, and Multiple Devices
Cookies facilitate multi-session analysis, but only, of course, if your visitors don’t delete cookies from their browsers. Similarly, a single individual who visits your site using multiple browsers (Firefox and Chrome, for example) will be tracked in Google Analytics as multiple separate users, as will one person who accesses your site on multiple devices.
How Does This Affect My Google Analytics Reports?
Cookie deletion impacts many Google Analytics reports. No data is lost per se; what we lose is the connection with previous user activity.
Let’s take inventory.
Cookie deletion increases the number of users (formerly called unique visitors) for the selected time period but does not increase session count unless the deletion occurs while the user is still active on your site.
New vs Returning
After cookie deletion, a returning user is recorded as new. The New Users count is always inflated relative to actual usage.
Frequency & Recency: Count of Sessions
If a user visits your site for the fifth time without deleting cookies, score one for Count of Sessions 5. After cookie deletion, we’re back to Count of Sessions 1.
Frequency & Recency: Days Since Last Session
A first-time visit or a visit after cookie deletion will automatically count as Days Since Last Session = 0.
Active Users (beta)
If a user visited 10 days ago, deleted cookies, and then visited again 5 days ago, that user will count once as a 14-day active user and once as a 7-day active user. If a user visits twice today and deletes cookies in between, that user will appear twice in 1-day active users through tomorrow.
Cohort Analysis (beta)
Cookie deletion will diminish metrics such as retention rate and goal completions, most notably in the higher day/week/month slots of the older cohorts, and will also increase counts in the newer cohorts.
By default, Google Analytics reattributes a direct returning visit to any more specific previous traffic source. Without a _ga cookie to identify the visit as returning, a direct visit will always count as Direct in the Channels and Source/Medium report.
After the user has deleted cookies, Google Analytics cannot tie in previous sessions as conversion assists.
Top Conversion Paths
Without a cookie to connect with previous touchpoints, the Top Conversion Paths report will display a greater number of conversions with shorter paths.
After cookie deletion, time lag to conversion would be reset to 0 days.
Model Comparison Tool
Cookie deletion can impact all attribution models except Last Interaction.
Cookie Considerations Continued
Apart from the impact of _ga cookie deletion on Google Analytics reporting, let’s review a few more points..
Demographics and Interests Reports
The Demographics and Interests reports in Google Analytics depend on the DoubleClick cookie. If users have deleted cookies, Google Analytics will not be able to access any Demographics and Interests data that would have been available for the user prior to cookie deletion.
Since cross-device tracking is in most cases based on user login during each session, it is not directly impacted by cookie deletion between sessions.
Remarketing / Retargeting
Remarketing, whether implemented through AdWords using Google Analytics remarketing audiences or through any other platform, depends directly on cookies. Cookie deletion will prevent remarketing ads from appearing to users and will also prevent the remarketing platform from crediting non-clickthrough (aka viewthrough) conversions.
A/B and Multivariate Testing
If user deletes cookies after an exposure to a split test on your site, the user will be considered new on the return visit and will be shown a randomly selected test variation and not necessarily the previous variation.
Have Cookies Become Stale?
We all hear rumors about an eventual alternative that would avoid the vulnerabilities of cookies as the linchpin of so many digital analytics and marketing technologies. For the time being, we can keep the limitations in mind but still enjoy the rich analytics data and marketing opportunities that cookies help to provide.