Captcha is the acronym for “Completely Automated Public Turing test to tell Computers and Humans Apart”, a type of challenge-response test used in computing to determine whether or not the user is human. A common type of captchas requires that the user type the letters of a distorted image, sometimes with the addition of an obscured sequence of letters or digits that appears on the screen. Captchas based on reading text — or other visual-perception tasks — prevent blind or visually impaired users from accessing the protected resource. These captchas also create barriers for the much larger number of people with learning disabilities involving text recognition (the visual perceptual problems often casually referred to as “dyslexia“).
Users do not need to have severe vision or learning disabilities to fail at least one attempt at the average captcha: for example, it’s difficult for most people to distinguish a distorted capital O from a distorted lowercase o, but some captchas require this. Moreover, optical captchas can be defeated by using OCR technology or simply by re-using the session ID of a known captcha image. Some variation to the traditional character recognition captchas has been introduced by image recognition captchas, which require users to identify simple objects in the images presented, with similar circumvention flaws. Visual captchas are a barrier for disability. As an alternative, some websites are providing audio captchas. This kind of antispam allows access to most blind users, but it still hinders many users, including those with slow Internet connections and/or poor audio hardware, non-native speakers of the language used in the audio file, users with auditory learning disabilities, and people who are both visually and hearing impaired.
In order to defeat spam while not harming users with disabilites, there are better solutions. WordPress users, for instance, can use the amazing Akismet plugin: when a new comment, trackback, or pingback comes to your blog it is submitted to the Akismet web service which runs hundreds of tests on the comment and returns a thumbs up or thumbs down. When the plugin catches something as spam it saves it in the database for 15 days in case you want to check it out manually and then automattically deletes it. In the unlikely event something gets incorrectly identified as spam you can correct it and it submits the “false positive” back to Akismet for analysis and improvement of their system. If a spam comment happens to get through and you mark it as spam within WordPress, it does the same thing. Akismet becomes more effective the more you use it. So, dear WordPress plugin developers, stop making captcha-based plugins!
WordPress users who want to protect their contact forms from spams, can additionally use this excellent plugin. If you want to send the form, just answer a question! As you can see, on this blog that question (customizable by the site owner) is tough enough to prevent spambots from polluting my mailbox 
It’s time to act up: fight spam and fight barriers to accessibility: don’t use captchas!
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