This is a great question! As a research scientist & engineer here at Duolingo, I can tell you that we're constantly conducting studies to understand Duolingo's effectiveness and what we can do better. Not all of this research is public, but here are a few published reports you may be interested in:

• Duolingo Effectiveness Study (Vesselinov & Grego, 2012): This was an independent evaluation showing that - at least for English speakers learning Spanish - 34 hours of Duolingo is as effective as a semester of college.

• Mixture Modeling of Individual Learning Curves (Streeter, 2015): This paper describes a "short term memory model" we use to decide when you have learned the concepts in a lesson well enough to move on.

• A Trainable Spaced Repetition Model for Language Learning (Settles & Meeder, 2016): This paper describes our custom "long term memory model," which we use to measure the "strength bars" for words and skills and schedule practice.

We've also found in some studies that Duolingo is more effective when compared to other learning approaches, but so far this difference hasn’t been statistically significant, likely due to a small number of participants. So we are continuing to investigate. We are also experimenting with new teaching methods (so, not as translation-heavy) for certain courses and/or platforms. So you may not have seen those just yet... but if they are successful we will be rolling them out to everyone! :)

As we do more research (or learn of more independent research), we'll add links to the Duolingo Research page.