I've just heard about "Asirra" and tried it. But, I am a little bit confusing about using. What is your choice for usability?
Asirra or CAPTCHA, and why?
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CAPTCHA is Completely Automated Public Turing test to tell Computers and Humans Apart. So, indeed, Asirra is (a type of) CAPTCHA... If you mean the warped text type of it, I think Asirra is not useful because:
I think best type/way is reCaptcha.
I had never heard of that one, interesting method.
However personally, I find it more user-friendly not to require a captcha (or assira) at all unless there are high enough indications that it might be spam, that way most users are not punished for wanting to interact with the website.
Personally I would avoid the use of any form of captcha wherever possible - they are a proven factor for form abandonment. There are some nice alternatives discussed here - the "honey pot" one in particular is a great idea.
We conducted a usability test (30 participants), and came to the conclusion that QuizCAPTCHAs rate first in terms of success rate and speed of solving, followed by ReCAPTCHA. Asirra was comparable to ReCAPTCHA, with the odd outcomings that especially elder participants had real difficulties telling cats and dogs apart. So I'd go for a math-based QuizCAPTCHA, or ReCAPTCHA, if you don't mind the Google part.
I have basically the same objection as spinodal - it requires extensive use of a mouse and even if you fail to mark a single picture correctly, it considers you a bot
there is a better alternative - somewhere in the middle between reCaptcha and Asirra
you can use the logic of Asirra and ask user a simple question, such as: what is the next day after Wednesday? - or - enter the next whole number before the number 5