The Facebook Test Posts feature finds the best performing variation of a post and publishes that post to the whole of the creator's audience.
The Facebook creator can select up to four variation posts, the duration of the test, and parameters deciding the winning post, these posts will circulate through a percentage of the audience and decide the winning post.
The feature is available below the 'Create Post' option in the sidebar of Creator Studio. A creator can set preferences as desired.
The feature is very similar to the learning phase for Facebook ads. The learning phase is a period when Facebook's delivery system learns about the best places and people to show an ad.
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While the new feature testing variations of a post can be used to get a general sense of what works and what doesn't, one shouldn't heavily rely on it to shape its content.
As the test only involves a set of users from your whole audience, and to test a post it should be tested with the maximum number of users, ideally the complete audience.
The more a post is shown, the results have a better sense of judgment. While one may be able to conclude through selected metrics and impressions that the winning post format can do well, one cannot be sure the post formats that didn't do well cannot perform.
Screenshot Source: Matt Navarra