it's gone noooooo!!!!!!!!!!!!
my start next lesson button is gone :( https://www.duolingo.com/comment/4313394
We measure the fraction of users who come back to Duolingo on consecutive days. This fraction was lower for users who had the button to the extent it is unlikely to be explained by statistical noise.
There's no causal explanation as to why a smaller fraction return, although you can dream up explanations such as new users should focus on revisiting previous lessons over advancing quickly in the tree. Is that really the case? We don't know because we weren't testing that hypothesis.
what happened to the Progress Quiz?
Mostly, you can find the answer to this kind of questions in
In https://support.duolingo.com/hc/en-us/articles/115002969903-Duolingo-s-New-Website Duolingo's staff wrote:
Progress quiz (we would rather focus efforts on creating other, more popular shop items)