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Comment by stingraycharles | Hacker News Reader

Wait. I must be missing something:

Since computers have trouble reading squiggly words like these, CAPTCHAs are designed to allow humans in but prevent malicious programs from scalping tickets or obtain millions of email accounts for spamming. But there’s a twist — the words in many of the CAPTCHAs provided by reCAPTCHA come from scanned archival newspapers and old books. Computers find it hard to recognize these words because the ink and paper have degraded over time, but by typing them in as a CAPTCHA, crowds teach computers to read the scanned text.

The way I understand this, is that the user is presented letters from archival newspapers and must type in the text he sees, and recaptcha uses that text to improve OCR. But doesn't that imply that recaptcha was unable to interpret the scanned text before ? If so, how can it then verify the correctness of the text the user types in ? If not, how exactly is this helping OCR ?

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  > But if a computer can't read such a CAPTCHA, how does the 
  > system know the correct answer to the puzzle? Here's how: 
  > Each new word that cannot be read correctly by OCR is 
  > given to a user in conjunction with another word for which 
  > the answer is already known. The user is then asked to 
  > read both words. If they solve the one for which the 
  > answer is known, the system assumes their answer is 
  > correct for the new one. The system then gives the new 
  > image to a number of other people to determine, with 
  > higher confidence, whether the original answer was 
  > correct.

http://recaptcha.net/learnmore.html

reCAPTCHA presents two words. One is known, the other is unknown. The idea is that if you type the known word correctly, then you've typed the unknown word correctly as well. It also shows the same unknown word to many people as a distributed accuracy check.

So reCaptcha uses a statistical accuracy and machine learning algorithms to compare the user input for the same word over hundreds of users. Thus by doing this they minimize the probability that a human who incorrectly deciphers a word skews their data.

Also reCaptcha's present two words. One of which is known, and used as a normal captcha, the other of which is an unreadable word. Thus if a someone passes the captcha word, its somewhat assumed that he also correctly typed in the unknown word because the words are presented in randomized order.

Finally, as a funny anecdote, Luis told me once of this time that they noticed there was a captcha farm in latin America paying pennies for poor latin american's to solve reCaptcha's for them in order to gain the ability to generate illegal spam. Luis started sending them whole sentences of reCaptcha's and eventually made it to full paragraph's. Ironically, the reCaptcha farm continued to solve these captcha's. He eventually stopped their activity, but he said the data they generated was some of the best he's ever gotten... :P

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