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Bertrand's box paradox is a veridical paradox in elementary
Calcul des Probabilités
'. There are three boxes: # a box containing two gold coins, # a box containing two silver coins, # a box containing one gold coin and one silver coin. A coin withdrawn at random from one of the three boxes happens to be a gold. What is the probability the other coin from the same box will also be a gold coin? A veridical paradox is a paradox whose correct solution seems to be counterintuitive. It may seem intuitive that the probability that the remaining coin is gold should be , but the probability is actually . Bertrand showed that if were correct, it would result in a contradiction, so cannot be correct. This simple but counterintuitive puzzle is used as a standard example in teaching probability theory. The solution illustrates some basic principles, including the
The problem can be reframed by describing the boxes as each having one drawer on each of two sides. Each drawer contains a coin. One box has a gold coin on each side (GG), one a silver coin on each side (SS), and the other a gold coin on one side and a silver coin on the other (GS). A box is chosen at random, a random drawer is opened, and a gold coin is found inside it. What is the chance of the coin on the other side being gold?
The following reasoning appears to give a probability of 1/2:
* Originally, all three boxes were equally likely to be chosen.
* The chosen box cannot be box SS.
* So it must be box GG or GS.
* The two remaining possibilities are equally likely. So the probability that the box is GG, and the other coin is also gold, is 1/2.
The reasoning for the 2/3 is as follows:
* Originally, all six coins were equally likely to be chosen.
* The chosen coin cannot be from drawer S of box GS, or from either drawer of box SS.
* So it must come from the G drawer of box GS, or either drawer of box GG.
* The three remaining possibilities are equally likely, so the probability that the drawer is from box GG is 2/3.
Bertrand's purpose for constructing this example was to show that merely counting cases is not always proper. Instead, one should sum the probabilities that the cases would produce the observed result.
Wason, Monty Hall, and Adverse Defaults
Estimating the Probability with Random Boxes and Names
a simulation {{DEFAULTSORT:Bertrand's Box Paradox Probability theory paradoxes Probability problems
Bertrand's box paradox is a veridical paradox in elementary probability theory
Probability theory or probability calculus is the branch of mathematics concerned with probability. Although there are several different probability interpretations, probability theory treats the concept in a rigorous mathematical manner by expre ...
. It was first posed by Joseph Bertrand
Joseph Louis François Bertrand (; 11 March 1822 – 5 April 1900) was a French mathematician whose work emphasized number theory, differential geometry, probability theory, economics and thermodynamics.
Biography
Joseph Bertrand was the son of ...
in his 1889 work Calcul des Probabilités
'. There are three boxes: # a box containing two gold coins, # a box containing two silver coins, # a box containing one gold coin and one silver coin. A coin withdrawn at random from one of the three boxes happens to be a gold. What is the probability the other coin from the same box will also be a gold coin? A veridical paradox is a paradox whose correct solution seems to be counterintuitive. It may seem intuitive that the probability that the remaining coin is gold should be , but the probability is actually . Bertrand showed that if were correct, it would result in a contradiction, so cannot be correct. This simple but counterintuitive puzzle is used as a standard example in teaching probability theory. The solution illustrates some basic principles, including the
Kolmogorov axioms
The standard probability axioms are the foundations of probability theory introduced by Russian mathematician Andrey Kolmogorov in 1933. These axioms remain central and have direct contributions to mathematics, the physical sciences, and real-worl ...
.
Solution
The problem can be reframed by describing the boxes as each having one drawer on each of two sides. Each drawer contains a coin. One box has a gold coin on each side (GG), one a silver coin on each side (SS), and the other a gold coin on one side and a silver coin on the other (GS). A box is chosen at random, a random drawer is opened, and a gold coin is found inside it. What is the chance of the coin on the other side being gold?
The following reasoning appears to give a probability of 1/2:
* Originally, all three boxes were equally likely to be chosen.
* The chosen box cannot be box SS.
* So it must be box GG or GS.
* The two remaining possibilities are equally likely. So the probability that the box is GG, and the other coin is also gold, is 1/2.
The reasoning for the 2/3 is as follows:
* Originally, all six coins were equally likely to be chosen.
* The chosen coin cannot be from drawer S of box GS, or from either drawer of box SS.
* So it must come from the G drawer of box GS, or either drawer of box GG.
* The three remaining possibilities are equally likely, so the probability that the drawer is from box GG is 2/3.
Bertrand's purpose for constructing this example was to show that merely counting cases is not always proper. Instead, one should sum the probabilities that the cases would produce the observed result.
Experimental data
A survey of psychology freshmen taking an introductory probability course was conducted to assess their solutions to the similar three-card problem. In the three-card problem, three cards are placed into a hat. One card is red on both sides, one is white on both sides, and one is white on one side and red on the other. If a card pulled from the hat is red on one side, the probability of the other side also being red is . 53 students participated and were asked what the probability of the other side being red were. 35 incorrectly responded with ; only 3 students correctly responded with .Related problems
Other veridical paradoxes of probability include: *Boy or Girl paradox
The Boy or Girl paradox surrounds a set of questions in probability theory, which are also known as The Two Child Problem, Mr. Smith's Children and the Mrs. Smith Problem. The initial formulation of the question dates back to at least 1959, when&nb ...
* Monty Hall problem
The Monty Hall problem is a brain teaser, in the form of a probability puzzle, based nominally on the American television game show ''Let's Make a Deal'' and named after its original host, Monty Hall. The problem was originally posed (and solved ...
* Three Prisoners problem
* Two envelopes problem
The two envelopes problem, also known as the exchange paradox, is a paradox in probability theory. It is of special interest in decision theory and for the Bayesian interpretation of probability theory. It is a variant of an older problem known ...
* Sleeping Beauty problem
The Sleeping Beauty problem, also known as the Sleeping Beauty paradox, is a puzzle in decision theory in which an ideally rational Epistemology, epistemic agent is told she will be awoken from sleep either once or twice according to the toss of a ...
The Monty Hall and Three Prisoners problems are identical mathematically to Bertrand's Box paradox. The construction of the Boy or Girl paradox is similar, essentially adding a fourth box with a gold coin and a silver coin. Its answer is controversial, based on how one assumes the "drawer" was chosen.
References
*Nickerson, Raymond (2004). ''Cognition and Chance: The psychology of probabilistic reasoning'', Lawrence Erlbaum. Ch. 5, "Some instructive problems: Three cards", pp. 157–160. *Michael Clark, ''Paradoxes from A to Z'', p. 16; *Howard MargolisWason, Monty Hall, and Adverse Defaults
External links
Estimating the Probability with Random Boxes and Names
a simulation {{DEFAULTSORT:Bertrand's Box Paradox Probability theory paradoxes Probability problems