The "Turing Test" is generally taken to mean an updated version of the Imitation Game Alan Turing proposed in his 1951 paper of the same name. An early version had a human and a computer, and a judge had to decide which is which. If they were correct less than 50% then the computer was considered "intelligent."

The current generally accepted version requires only one contestant, and a judge to decide whether it is human or machine. So yes, sometimes this will be a placebo, effectively, if we consider a human to be a placebo.

Your first and fourth questions are related - and there are no strict guidelines. If the computer can fool a greater number of judges then it will of course be considered a better AI.

The University of Toronto has a validity section in [this paper on Turing][1], which includes a link to [Jason Hutchens' commentary][2] on why the Turing test may not be relevant (humans may also fail it) and the [Loebner Prize][3], a formal instantiation of a Turing Test .


  [1]: http://www.psych.utoronto.ca/users/reingold/courses/ai/turing.html
  [2]: http://ciips.ee.uwa.edu.au/Papers/Technical_Reports/1997/05/Index.html
  [3]: http://www.loebner.net/Prizef/loebner-prize.html