Why is the Predicted Audience Size of my Segment different to the Live Audience Size?

While most of the time the numbers will agree with each other, in some specific cases differences in numbers between these can arise, as the logic between them differs.

Predicted Audience Size:

This number is an estimate of how many users during the last 30 days have satisfied your segment criteria. This number should be used before the segment is live to predict the segment size, and for 30 days after a segment has been set live. It should also be used for time-restricted segments, where the time restriction is < 30 days.

Live Audience Size Estimator:

This is the number of users in this segment who have been active on your page within the last 30 days. This number should be used for non-time-restricted segments after 30 or more days since a segment has been created, as well as for long-running segments with no time constraints.

There are a few cases when these numbers will differ.

Your segment has been live for less than 30 days

Use the Predicted Audience Size Estimator

The Live Audience Size Estimator only begins counting users once a segment is set live, whereas the Predicted Audience Size looks back in history on user's behaviour even if a segment doesn't exist yet. Due to this difference, if a segment has been live for under 30 days, the Predicted Audience Size will be larger, as the Live Audience Size Estimator hasn't fully populated yet.

Your segment has a time constraint that is less than 30 days

Use the Predicted Audience Size Estimator

For example, if your segment includes a time constraint like "within the last 7 days", this will be reflected in the Predicted Audience Size Estimator but not in the Live Audience Size Estimator. Due to this, the Live Audience Size Estimator will show you the number of users who have been in the segment over the last 30 days, and the Predicted Audience Size Estimator will show you the users who have fallen into the segment in the last 7 days. The Live Audience Size Estimator can be significantly larger in this case

You have a long-running segment with no time constraints

Use the Live Audience Size Estimator

For example, if your segment is "users who have read a football article" with no time restrictions, the Predicted Audience Size will return the number of users who have read a football article in the last 30 days. The Live Audience Size will return the number of users who have ever read a football article, and were active on your site in the last 30 days.

You have changed the definition of your segment recently

Use the Predicted Audience Size Estimator

If you have recently changed the definition of your segment, this change will be reflected in the Predicted Audience Size, but not in the Live Audience Size. Live Audience Size will include the users who have fallinen into the segment under your previous segment definition.

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