Completely Automatic Public Turning test to tell Computers and Humans Apart, or CAPTCHA, is a security measure that guards a system from exploitation by the discrimination between a real human being and an automated computer program via the method of presenting to the unknown user the challenges that are hard for computer yet easy for human. Focusing on text-based CAPTCHA, this study conducted an experiment to study the effect of age groups and distortion types on the CAPTCHA task. Twenty-four participants were recruited to take part in the experiment, where twelve of them were in the senior group and twelve in the young group. Participants were observed to use three general steps: recognition, rehearsal, and motor response. With the inevitability of the security measure and the increasing population of senior netizens, this study has important implications for the design of CAPTCHA systems.
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