Provide alternate CAPTCHAs in the form of logic tests
Make sure alternative text CAPTCHAs are provided instead of image or audio CAPTCHAs
Regular CAPTCHAs, whether they rely on the users’ ability to decode images of distorted text or scrambled audio files, are inherently flawed since they force users to struggle with a format which, by its very nature, is meant to be difficult to solve (some would say almost impossible). As the technology developed to automatically decode CAPTCHAs is evolving at a pace that is much faster than the users’ ability to keep up, there comes a point where a lot of users just cannot successfully solve those tests. As anyone who has had to refresh a CAPTCHA test a few times before they were offered one they could solve will tell you, image-based or audio-based CAPTCHAs are a failed approach to a false problem. In order to determine whether or not a user is really a human being and not a spamming bot, authors should rely on text-based CAPTCHAs instead.
To do so, simply make sure that CAPTCHAs used in pages take the form of written logical questions, which are yet complex enough not to be easily figured out by computers. [...]