Privacy
Cross-device tracking
Also called stitched users.
Cross-device tracking is linking the same person across browsers or devices using a shared key. The usual key is a login, and sometimes it is a statistical match.
How it is measured
The reliable method is deterministic: the person signs in on both devices and the site sends the same user ID. The weaker method is probabilistic, guessing from IP, time, and behavior.
To measure it, compare counts with and without the stitching. If 1,000 client IDs collapse to 780 user IDs, you have an estimate of how many people used more than one browser.
Worked example
A learning platform sees 1,000 client IDs among logged-in learners this month. After joining on user ID, there are 780 people. 220 of the IDs belonged to second devices.
Course completion by person rises from 31 to 40 percent, because someone who started on a phone and finished on a laptop is now one person.
How it differs
Cross-device tracking joins devices into one person. A user ID is the key that makes the joining possible when people sign in. Cross-device is the result; the user ID is one ingredient.
Common errors
Joining on a hashed email and calling it anonymous. Using probabilistic matches as facts. Stitching without a notice. Expecting it to work for visitors who never sign in. Merging two different people on a shared family computer.
In practice
Use it only where users sign in and expect it. Tell them in your privacy notice. Keep the match key out of reports.