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 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.
Websites that are still using the classic Google Analytics tracking code write the __utma, __utmb, __utmc, and __utmz cookies to your browser. If classic GA is configured for user-scope custom variables, the __utmv cookie is also written. Universal Analytics, on the other hand, drops only the _ga cookie into your browser.
As a note, if you have not yet migrated to Universal, you should plan to do so no later than April 2016, and sooner if possible. If you implemented Google Analytics after April 2, 2014, you’re already using Universal and do not need to migrate.
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.
If a visitor accesses your website in a private/incognito browser window, Google Analytics will still track activity during the session, but the _ga cookie will not persist past the session. Any return visit will therefore be attributed to a new user.
The opt out / do not track plugins that Google and third parties provide for Chrome, Firefox, Internet Explorer, and Opera, on the other hand, prevent Google Analytics from tracking any user activity in that browser. You could build an opt-out selection directly into your site and call window['ga-disable-UA-XXXX-Y'] = true; in the Google Analytics API as prompted by the user, if this degree of tracking control was ever required in your Google Analytics implementation.
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.
After cookie deletion, a returning user is recorded as new. The New Users count is always inflated relative to actual usage.
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.
A first-time visit or a visit after cookie deletion will automatically count as Days Since Last Session = 0.
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.
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.
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.
Cookie deletion can impact all attribution models except Last Interaction.
Apart from the impact of _ga cookie deletion on Google Analytics reporting, let’s review a few more points..
Because mobile apps do not use cookies for identifying return visitors, cookie deletion is not relevant for app tracking.
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.
As a note, the Demographics and Interests reports are not enabled by default; they require a small customization in the Google Analytics tracking code (or the Google Analytics tag in Google Tag Manager or another tag management system) as well as a privacy policy revision, as discussed below.
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, 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.
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.
If you’re using Google Analytics on your site, the terms of service require you to display a privacy policy similar to example below or to link to the privacy & terms for partners.
This website uses the first-party Google Analytics cookie only to track your return visits but not to identify you personally by name, email, or any other data.
If you’ve enabled display features (and are thereby using the DoubleClick cookie) for Demographics & Interests reporting, AdWords retargeting, or DoubleClick integration with Google Analytics Premium, you must display additional verbiage.
For details, see the Privacy section in the Terms of Service and Policy Requirements for Google Analytics Advertiser Features.
If you’re operating from within the European Union, your member state’s interpretation of the EU ePrivacy Directive (aka cookie law) may require additional provisions for privacy.
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.
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