What is CAPTCHA Usability

Completely Automated Public Turning Test to Tell Computers and Humans Apart (CAPTCHA) is a security mechanism that is used by online communities to block spam and hacking. Today, there are a large number of sophisticated CAPTCHAs that are robust, however, most of them are unusable. This chapter describes a study to examine types and characteristics of Text CAPTCHAs used by top 50 online communities. Furthermore, the study the first of its type to develop a scale to measure usability of text CAPTCHA and to examine the relationships between dimensions of CAPTCHA and perception of usability and satisfaction with registration process in online communities. Factor analysis and equation modeling study suggests that text-based CAPTCHA can be measured on four reflective dimensions of: (1) content; (2) visual layout; (3) distortion; and (4) service in which correlate significantly to users' sanctification with online community.