cookieMonsterFirst, a little overview of the purpose of cookies in terms of analytics (not milk!).

Simply put, in the analytics world, cookies are used to track and count unique visitors.  Cookies come in two flavors, first party and third party.  First party cookies are set by the same domain as the page the user is currently viewing, while third party cookies are set by a domain different from the page the user is currently viewing.  As browsers add features to support greater Internet privacy, the acceptance of third party cookies is diminishing. 

When cookies are rejected, Adobe uses a fallback method to determine a unique visitor. The fallback method varies depending on the analytics code version implemented on your web site.  Broadly speaking, there are three possible fallback methods, the original method using IP address and user agent string, the new fallback method using the s_fid first party cookie, and finally, the option to go with a true first party cookie.   This blog post outlines all three methods to help you make the right cookie decision for your analytics needs (with or without milk!).

Third Party Cookie – Original Fallback Method:

All code versions previous to H.25.3 use the original fallback method to handle cookie rejection.   When a third party or first party cookie is rejected by the browser, Adobe uses a combination of user’s IP address and the user agent string to identify visitors.

Things to keep in mind regarding visitor count:

  • Multiple users may be counted as a single visitor:  If two users are on the same network and use the same browser, which rejects third party cookies, then both users will appear to have the same IP address and potentially the same user agent string, resulting in two separate users being counted as a single visitor.
  • A single visitor may be counted multiple times:  If a user’s IP address changes halfway through the visit or the user agent changes in the future, a new visit and visitor will be counted for the same user.

Third Party Cookie – New (s_fid) Fallback Method:

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For analytics code versions from H.25.3 onwards, the code contains a new fallback visitor identification method.  If the visitor id cookie is rejected, the code will set a first party cookie using the site’s current domain.  The new first party cookie, s_fid, is created and set to a randomly generated unique ID with an expiration of two years.  If an s_fid, first party cookie cannot be set, then the Adobe solution will use the original fallback method of IP address and user agent combination.

Things to keep in mind regarding visitor count:

  • Visitor accuracy matches true first party cookies:  This solution sets a first party cookie and its rejection rate is as low as any first party cookie implementation.
  • Visitor Uniqueness:  The methodology used to generate the s_fid value does not guarantee a unique value for each visitor. Factors that impact uniqueness include the volume of s_fid values set across the site and the number of concurrent hits from visitors who arrive at the same millisecond. It’s important to note that the odds of this are very low, and you shouldn’t notice an even remotely significant difference between visitor counts using this methodology and using CNAME first-party cookies.
  • Decrease in visitors after code update:  If you update your analytics code to H.25.3 or higher, you may see an overall decrease in your visitor count because of the decrease in overall cookie rejection.

First Party Cookie:

For all versions of the analytics code, first party cookie tracking is the recommended approach.  First party cookie implementations provide the most accurate count of visitors because of the very low cookie rejection rate, as well as the guarantee of a unique and persistent visitor id value.

Things to keep in mind regarding visitor count:

  • Decrease in visitors after update from original:  If you choose to transition from the original fallback method to first party cookie, you will see an overall decrease in unique visitor count resulting in a more accurate report.
  • Increase in visitors after update from s_fid:  If you choose to transition from the s_fid fallback method to true first party cookie, during the transition phase you will see an increase in visitor count because Adobe will disregard the s_fid value and instead set a new cookie value resulting in visitors being recounted.

Again, it is our recommendation to always use first party cookie tracking when possible.  First party cookies will always give you the most accurate visitor count.  However, if you need to use third party cookies, the new s_fid fallback method will provide you with a good degree of accuracy. Now that you know how each method works, I hope you can figure out which approach best meets your needs!

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