Thesis (M.Sc.)--Chulalongkorn University, 2012

CAPTCHA is an important technique for protecting computer systems from malicious software accessing computer systems through the Internet. CAPTCHA techniques, however, pose many difficulties to users. Users usually gain entry to systems through layers of questions in various presentation styles that are both time-consuming and cumbersome. Therefore, the usability of CAPTCHA is a significant issue that all designers must concern. Unfriendly CAPTCHA presentations can hinder authorized users to gain access to the required system. In this research, many factors are studied to determine factors that affect the effectiveness of using CAPTCHA. The study was performed with three nationalities: Bhutanese, Indian and Thai. This study shows some dissimilarities between these nations that can have impact in the success of using CAPTCHA.