the lines of "is this really a human?". Describing such a situation as a "reverse Turing test" typically occurs retroactively. There are also cases of accidental reverse Turing tests, occurring when a programmer is in a sufficiently non-human mood that his conversation unintentionally resembles that of a computer. In these cases the description is invariably retroactive and humorously intended. The subject may be described as having passed or failed a reverse Turing test or as having failed a Turing test. The latter description is arguably more accurate in these cases; see also the next section. Failure by control subjects Since Turing test judges are sometimes presented with genuinely human subjects, as a control, it inevitably occurs that a small proportion of such control subjects are judged to be computers. This is considered humorous and often embarrassing for the subject. This situation may be described literally as the human "failing the Turing test", for a computer (the intended subject of the test) achieving the same result would be described in the same terms as having failed. The same situation may also be described as the human "failing the reverse Turing test" because to consider the human to be a subject of the test involves reversing the roles of the real and control subjects. Judgement by computer The term "reverse Turing test" has also been applied to a Turing test (test of humanity) that is administered by a computer. In other words, a computer administers a test to determine if the subject is or is not human. Such procedures, called CAPTCHAs, are used in some anti-spam systems to prevent automated bulk use of communications systems. The use of captchas is controversial. Circumvention methods exist that reduce their effectiveness. Also, many implementations of captchas (particularly ones desired to counter circumvention) are inaccessible to humans with disabilities, and/or are difficult for humans to pass. Note that "CAPTCHA" is an acronym for "Completely Automated Public Turing test to tell Computers and Humans Apart" so that the original designers of the test regard the test as a Turing test to some degree. Judgement of sufficient input An alternative conception of a Reverse Turing Test is to use the test to determine whether sufficient information is being transmitted between the tester and the subject. For example, if the information sent by the tester is insufficient for the human doctor to perform diagnosis accurately, then a medical diagnostic