User Filter

A user filter is a condition applied to people's properties or behaviour that restricts a report to just that subset of users.

A user filter is not the same thing as an event filter, and confusing the two is the most common reporting mistake here: an event filter discards non-matching event rows, while a user filter removes the whole person along with every event they produced. As a result, users who purchased once and purchase events give two very different numbers. The second trap is forgetting the denominator: filtering shrinks both the numerator and the base of any rate, so the resulting percentage cannot be compared with the unfiltered one unless both bases are stated.