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Source: Google & TED
Published: December 2011

Massive Scale Online Collaboration

Circulated: August 28, 2019

CAPTCHA (2003)

  • A challenge-response test used in computing to determine whether or not the user is human.
  • Letters and numbers distorted as squiggly characters (e.g. dG3T).
  • 200M per day as of 2009 at 10 seconds each, totaling 500K man-hours everyday.

reCAPTCHA (2007)

  • A company that uses CAPTCHA to perform useful tasks like preserve books, improve maps, and build machine-learning training data.
  • Example of a task: digitize scanned books by asking humans to type words that the computer could not recognize.
  • Acquired by Google in September 2009.
  • Why you’ll often get 2 words.
  • Verify: a word the system knows the answer to and assumes you’re human if you can confirm it.
  • Decode: a word the system doesn’t know. It won’t grade you for this answer. You and others are asked the same word which contribute to the computer’s knowledge by crowdsourcing the answer.


Invisible reCAPTCHA (2018)

  • Doesn’t present any human verification visually.
  • Monitors user actions (e.g. cursor movements, rate of scrolling).
  • Returns a score which represents the probability if the user is a human or a bot.

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