I've notised that Stack Overflow only presents me with a captcha occasionally. Does anyone know if it has a good algorithm for sensing if I might be a robot or, instead if it's just random.
Reducing reliance on captchas is obviously a good thing.
Captcha algorithm
2.1k Views Asked by Rog At
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It appears when you post (or edit) the same entry or an entry in the same question many times. It had appeared to me when I edit twice in less than a minute
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My impression is that SO monitors user's activity and presents captcha in case of suspicious beaviour, for example in case of frecuent posts or comments.
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I've never been confronted with a Captcha at SO because I used a google account and have not given it any reason to suspect me of being a bot I guess. If you don't create local logins you save yourself the trouble of Captcha and storing passwords.
EDIT I have now seen several captchas on the site, don't really have any idea what triggers them.
Stackoverflow.com uses reCAPTCHA. And, I too have noticed that when you try to edit the same question many times in a gap of a few seconds, it displays the captcha verification page.
Most of the CAPTCHA generation algorithms are strong enough. This article describes the factors influencing a CAPTCHA's strength.