It is set at a particular score, which is approximately 20%. As the number of quizzes online grows, the figure is changed periodically. At the moment, it is 22,000. which is 18.6% of the number of quizzes currently online. At some point, the threshold will be raised again, when the percentage included gets too far below the target.
(Editor in Humanities, Literature and Books For Children)
That's all, folks!