Why do some users appear in two segments that should be mutually exclusive (e.g., users who performed an action vs. users who never performed it)?

This can happen when a single user has multiple devices. By default, segment conditions are evaluated for each device individually, not for the user as a whole.

For example, a user might have two devices:

• Device A: the user has performed 'Event X'.
• Device B: the user reinstalled the app or logged in on a new phone and has not yet performed 'Event X' on this device.

In this scenario:

• A segment for users who performed 'Event X' will include this user because of Device A.
• A segment for users who never performed 'Event X' will also include this user because of Device B.

This results in the same user appearing in two logically opposite segments.

Solution

To evaluate segmentation against the user's overall activity across all their devices, switch the segment to user scope with the Include these users' other devices checkbox.

1. Open the segment in Audience > Segments and click to edit it.
2. Tick Include these users' other devices. It sits above the conditions and applies to the segment as a whole — there is no per-condition toggle.
3. Save the segment and click Update segment size to recalculate.

Behaviour of the option:

• Ticked — if any of the user's devices meets the conditions, all of that user's devices across platforms are included in the segment.
• Unticked (default) — only the devices that meet the conditions are included.

Tick it on both segments of a mutually exclusive pair, otherwise the two segments will still be evaluated on different scopes and can overlap.