What percent of all active IP + User Agent combinations are not unique?

ubuntu_ulysses

It's fairly common practice to define a unique user by a 1-to-1 hash of IP + User Agent. But not all unique users have a unique IP + User Agent combination. Thus: collisions - events misattributed to users who did not fire those events. This is especially problematic when trying to track unique users who use similar machines and settings all under the same roof of a large organization like a school or enterprise. My question is: What is the...

Originally asked by samthebrand on Stack Exchange (webmasters) (CC BY-SA 3.0).

uphilltech

Great question — answering as the UphillTech team:

It is not clear what your goal is. You do not tell us. If you are trying to identify unique users, you are using two of the most unreliable data elements possible. Yes IP addresses are often used, but in conjunction with other data elements. Here is what you are missing. IP addresses are often dynamic and can change with every session. As well IP addresses can be shared amongst many users. Also consider the limited number of web browsers and the user agents provided. It is extremely common that IP Address + User Agent will result in collisions as you suggest. It is extremely probable in fact. Also consider the amount of times that a user agent is bogus. It seems these days that user agents...

Full answer by closetnoc: https://webmasters.stackexchange.com/a/63037 (CC BY-SA 3.0).

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