Duolingo has a unique approach to measuring success, focusing on "D1 retention," or the percentage of users who return the day after signing up. Starting with a 15% D1 retention rate at launch, Duolingo has since improved to over 50% today by prioritizing this metric.

One factor that contributes to user engagement is the use of gamification

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Serving quizzes in real-time and gating continued learning based on your results, and recommending additional resources based on where you struggle.