I’ve been thinking about how to track unique users today. These are my so-far-unorganized thoughts. Please comment!
You can’t track users by cookie alone for a couple of reasons: 1) they might use multiple computers, 2) they might delete cookies, 3) multiple users might share the same computer (same account=same cookie)
You also can’t track users by IP address alone for some more reasons: 1) they might be using a mobile device or portable computer that moves from IP to IP, 2) there could be multiple machines passing through a single gateway IP (i.e. LAN NAT).
However, if you combine the cookie/IP information together, you can start to address some of these issues. Let’s assume you have some webserver logs that minimally contain <IP address "A">, <cookie ID "C">, <timestamp T> triples.
=== time ===>
PATTERN 1:
C1-A1 --- ---
C1-A2 --- ---
C1-A3 --- ---
PATTERN 2:
C1-A1 ------
C2-A1 ------
C1-A2 ------
PATTERN 3:
C1-A1 -----!
C2-A1 -----!
C3-A1 ------
PATTERN 4:
C1-A1 ------ ------
C2-A1 ----------
PATTERN 5:
C1-A1 ----- -----
C2-A1 ----- ---
This matrix indicates the compatibility of each of the patterns (P1-P5)
with several different classes of cookie/IP address combination that we
might want to detect.
patterns
P1 P2 P3 P4 P5
profiles --------------------
multiple users per IP | - + - + +
multiple users per cookie | - - - - -
multiple IPs per user | + + - - -
multiple cookies per user | - - + - +
cookie deletion | - - + - -
"permanent" IP change | - + - - -
Note that none of these patterns gives any indication for the “multiple
users per cookie” profile. To assess if there is more than one
user/cookie, you might want to look at the context in which you’re
observing the cookie. Consider attributes like (timezone corrected)
time-of-day, day-of-week, type of content being viewed.
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