You can only sign up to Mechanical Turk in the US and India. This is presumably due to the legal complexity of paying users.
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What happens if you do a survey on Mechanical Turk asking programmers what semantics they expect of their languages?
https://blog.brownplt.org/2017/07/06/crowd-pl-design.html has the answer! Direct link to PDF: https://cs.brown.edu/~sk/Publications/Papers/Published/tpk-crowdsource-lang-design/paper.pdf
Thread.
A major problem with captchas is that OCR is good enough to fool it some of the time. Will machine learning do the same to Mechanical Turk?