Which unexpected metric did the Khan Academy marketing team discover was most strongly correlated with long-term user retention for their advanced mathematics courses?

In the world of mathematics education and learning tools, marketing analytics play a crucial role in product development and distribution. Companies invest heavily in understanding how their educational tools perform in the market and which metrics truly indicate success. This trivia question explores how a leading mathematics learning platform made a surprising discovery about their product's performance indicators.

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Time spent on first three practice problems (not total session time)
Number of achievements/badges earned in the first month
Frequency of video playback speed adjustments
Completion percentage of suggested supplementary materials