Does Duolingo Work?

    Duolingo’s design largely relies upon a system called 
    “spaced repetition,” a technique in which learned information 
    is repeated at regular (usually short) intervals

Aside from the author's other criticisms, she just takes it on faith that Duolingo actually implements a proper SRS. I'm not sure they do. The other apps and webapps I have used seems seem to have much better Spaced Repetition algorithms.