Reliance on cookies in web site user tracking

In the field of web analytics, one of the permanently hot topics is how to track a user’s activity on a web site accurately with the information that the technology available to us is able to provide. There are two main aims:

  • Tracking a user from their entry point into a site until they leave (known as a user’s ‘session’
  • Recognising the same user then they return to the site at some time in the future (to recognise them as a ‘visitor’)

This has been raised to the forefront again recently by Eric Peterson’s study for Jupiter that indicates that a high proportion of users (in the US, where the study was performed) regularly delete cookies from their computers.

Cookies are the principle, and still most reliable,form of tracking technology used by the major web analytics companies. These are used not only to track sessions, but also to recognise repeat visits by the same user.

If cookies are being deleted as regularly as the Jupiter report suggests then this would have the following implications for any web analytics that relies on cookie-based tracking:

  • If a user deletes a tracking cookie whilst in the middle of a session then their visit will not be tracked correctly, i.e it will be tracked. as two (or more) visits rather than one. Also, any conversions may be attributed to the wrong source.
  • If a user deletes a tracking cookie between sessions then they will not be recognised when they return to the site at a later date

Interms of what these would mean to site performance reports some of the following should illustrate why this is such an important issue for website owners:

  • Drop-off points will be incorrectly highlighted. Although this may not be statistically significant it will make it more difficult to identify trends.
  • The number of sessions reported will be higher than it is in reality
  • Conversion statistics may appear lower than they really are. One session may be incorrectly identified as two (or more), whereas only one will lead to a conversion
  • Conversions may then be assigned to the wrong source, possibly leading to incorrect decisions to abort or continue campaigns
  • Numbers of distinct visitors will appear higher than they really are
  • Repeat visits will appear lower, which may lead to unfounded doubts about the ‘stickiness’ of the site’s content.

The industry as a whole is trying to find ways to raise accuracy levels by other means but this brief look at the implications of cookie deletion when using a cookie-reliant tracking product shows what an important issue this is.

Jupiter study about cookie deletion rates

Nielsen Corroborates Jupiter’s Cookie Deletion Report

Clickz commentary on the jupiter report

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