I was recently asked about a specific question from the October 1999 LSAT (O99, LR1, #7) on our LSAT Discussion Forum. This question, known as Debbie's Magic Act, can be summarized as follows:
Debbie has a magic act where she identifies a card chosen randomly from a deck, without ever looking at the card beforehand. A skeptic examined the process, and conducted three separate tests. In the first test, he made a video of her selecting the correct card, and after examining it he determined she did not perform sleight-of-hand. In the second test he gave her his own deck of cards and she again succeeded, so the skeptic concluded it wasn't a stacked deck. In the third test, the skeptic selected the card on his own, and on that basis ruled out the possibility that a plant in the audience had been used. On the basis of these three tests, the skeptic concludes that none of the three methods—sleight-of-hand, stacked deck, or planted volunteer—had been used by Debbie to perform the magic trick. This stimulus is then followed by a Flaw in the Reasoning question.
The reason this question interests me is that a large number of students select the same incorrect answer, and are very confident they are correct even after reading the problem again. Even after looking at the right answer these students often do not necessarily understand where things went awry with the answer they chose. It's a tricky question! The trap used to create this confusion is one I sometimes refer to as the Natural Question Error, and the makers of the LSAT have preyed upon it in many Logical Reasoning questions. It's a type of Shell Game error, and if you study for the LSAT long enough, you will at some point make the very same mistake. So let's break this problem down and see how we can avoid this error.