Details on personalized learning at Duolingo

There's a new, great, long article on how Duolingo's personalized learning algorithms work, "How Duolingo's AI learns what you need to learn".

An excerpt as a teaser:

When students are given material that’s too difficult, they often get frustrated and quit ... [Too] easy ... doesn’t challenge.

Duolingo uses AI to keep its learners squarely in the zone where they remain engaged but are still learning at the edge of their abilities.

Bloom’s 2-sigma problem ... [found that] average students who were individually tutored performed two standard deviations better than they would have in a classroom. That’s enough to raise a person’s test scores from the 50th percentile to the 98th

When Duolingo was launched in 2012 ... the goal was to make an easy-to-use online language tutor that could approximate that supercharging effect.

We'd like to create adaptive systems that respond to learners based not only on what they know but also on the teaching approaches that work best for them. What types of exercises does a learner really pay attention to? What exercises seem to make concepts click for them?

Great details on how Duolingo maximizes fun and learning while minimizing frustration and abandons, even when those goals are in conflict. Lots more in there, well worth reading.