I don't know if we are talking at cross purposes, or if I missed it, but this paper: http://elie.im/publication/text-based-captcha-strengths-and-weaknesses does not try to answer my question.
What I want to know is "*How many humans get turned away from editing Wikipedia by a difficult captcha?*"
The same authors have: http://elie.im/publication/how-good-are-humans-at-solving-captchas-a-large-s... which is closer to what I want to know. They show humans solving different text based captures with an accuracy rate of 70% to 98%. Unfortunately, Wikipedia was not one of the captcha schemes they used in that study, and they don't attempt to measure how many people try again if they fail.
If 2% of people fail on the first try but 90% of the fails reattempt and only 1% fail a second time that's an inconvenience, but probably worth it if it reduces the inconvenience of spam.
If 30% of people fail on the first try and 90% of them give up and never try to edit again, that's a disaster.
Luke
On Wed, Jan 23, 2013 at 12:24 AM, Federico Leva (Nemo) nemowiki@gmail.comwrote:
Luke, "we do not know how many humans are being turned away by the difficulty": actually we sort of do, that paper tells this as well. It's where their study came from, and gives recommendations on what captcha techniques are best for balancing efficacy with difficulty for humans. We don't seem to be following any (except waving, which, they say, shouldn't be used alone). Then, I'm not qualified to say if their recommendations are the best and I've not searched other studies, but it's not correct to say that we start from zero or that we have to study by ourselves (an unreasonable requirement that implies we'll never change anything until we'll be forced to make our wikis read-only due to spam, as many MediaWiki users before us).
Nemo
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