SEOMoz has posted some original research on effect of CAPTCHAs on conversion rates:
With CAPTCHA’s on, SPAM and failed conversions accounted for 7.3% of all the conversions for the 3 month period. With CAPTCHA’s off, SPAM conversions accounted for 4.1% of all the conversions for the 3 month period. That possibly means when CAPTCHA’s are on, the company could lose out on 3.2% of all their conversions!
In other words, a significant proportion of frustrated customers simply abandon their attempts to get past the CAPTCHA. (And, notably, some spam still got through!)
We’ve blogged before about the usability problems of CAPTCHA-based forms, and it’s good to see some real-world data measuring those effects.