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The term reverse Turing test has no single clear definition,
but has been used to describe various situations
based on the Turing test in which one or more of the roles have been
reversed between computers and humans.
Conventionally, the Turing test is conceived as having a human judge
and a computer subject which attempts to appear human. Critical to
the concept is the parallel situation of a human judge and a human
subject, who also attempts to appear human. The intent of the test is
for the judge to attempt to distinguish which of these two situations
is actually occurring. It is presumed that a human subject will always
be judged human, and a computer is then said to "pass the Turing test"
if it too is judged human. Any of these roles may be changed to form a
"reverse Turing test".
Reversal of objective
Arguably the standard form of reverse Turing test is one in which
the subjects attempt to appear to be a computer rather than a human.
This is considered an inherently humourous situation.
A formal reverse Turing test follows the same format as a Turing test.
Human subjects attempt to imitate the conversational style of a
conversation program such as ELIZA. Doing this well involves
deliberately ignoring, to some degree, the meaning of the conversation
that is immediately apparent to a human, and the simulation of the kinds
of errors that conversational programs typically make. Arguably unlike
the conventional Turing test, this is most interesting when the judges
are very familiar with the art of conversation programs, meaning that
in the regular Turing test they can very rapidly tell the difference
between a computer program and a human acting normally.
The humans that perform best (some would say worst) in the reverse
Turing test are those that know computers best, and so know the types
of errors that computers can be expected to make in conversation.
There is much shared ground between the skill of the reverse Turing
test and the skill of mentally simulating a program's operation in the
course of programming and especially debugging. As a result,
programmers (especially hackers) will sometimes
indulge in an informal reverse Turing test for recreation.
An informal reverse Turing test involves an attempt to simulate a computer
without the formal structure of the Turing test. The judges of the
test are typically not aware in advance that a reverse Turing test is
occurring, and the test subject attempts to elicit from the 'judges'
(who, correctly, think they are speaking to a human) a response along
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 humourously 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 humourous,
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.
It may also be described as the human "passing the reverse Turing test",
on the grounds that if the real and control subjects have been exchanged
then the objective should also change to maintain the structure of the
Turing test.
Reversal of real and control subjects
When a Turing test is applied to a mixed group of human and computer
subjects, the computers are the nominal subjects. However, the humans
are judged in exactly the same way, and so their Turing test scores can
also be calculated.
This is another way of viewing the situation described in the previous
section.
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. As of 2004,
such procedures are used in some anti-spam systems to prevent
automated bulk use of communications systems. Their use is controversial.
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