Usability aspects of HCI in the design of CAPTCHAs

Usability Engineering is one of the key aspects of Human Computer Interaction designs. Some of the characteristics of a usable system are learnability, rememberability, efficiency and effectiveness of use, etc. Literature establishes the significant impact of the format of input, feedback, visibility to enhance the usability of HCI systems. This paper focuses on usability aspects in the domain of CAPTCHAs, which play a vital role in differentiating human beings from automated programs in various secure / internet based applications. Different types of CAPTCHAs, principles behind their evolution, safety and demerits of each are studied. Proposals to extend / augment the existing CAPTCHA designs are also presented in this paper.