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.