uppermost side of the coin is known by the tosser (and caller).
If this is not the case, the true 50:50 probability of the result
prevails. But if it is known, the side of the coin that starts off
face up is more likely to end that way up because that side
spends more time facing up during the flight than does the
opposite side. However, because of the large number of tosses
required to detect this difference (shown to be 250 000
tosses), this apparent difference is generally irrelevant.
Coins have different raised profiles on each side. In theory,
one side could be more weighted, thus making the coin toss
unfair. This could be achieved by tampering with a coin. With
the introduction of the Euro, Polish statisticians claimed that
the 1 € coin (from Belgium), when spun on a surface, came
up heads more often than tails.3 This report resulted in a
tongue-in-cheek warning in the British press to teams playing
against Belgium in the forthcoming soccer World Cup.4 Of
250 spins, 56% came up heads. However, independent statistical
analysis showed that random variation could produce
such scatter even with an unbiased coin given this number of
spins (acceptable range 43.8%–56.2%). Other research suggests
that even if a grossly weighted coin is used (1 side lead,
the other balsa wood), no significant bias shows up.3
The usual method of tossing a coin should not be taken for
granted. Gary Kosnitzky, a Las Vegas magician adept in the
art of coin magic, can reputedly manipulate coin tosses to give
a predictable outcome by use of a method that gives the illusion
of a spinning coin. When his coin “spins” in the air, it is
not rotating about its axis but is instead fluttering or wobbling
and not turning over. This gives the illusion of a normally
spinning coin, yet gives a predictable result if caught in the
palm of the hand.5 Admittedly, this is probably an art more
practised by the gambler than the researcher, but it serves as a
reminder that an apparently fair situation can be manipulated.
The most compelling finding that raises concerns about the
validity of the coin toss come from the use of mechanical coin
flippers, which can be made to impart exactly the same initial
conditions for every toss, namely the starting position, velocity
and force. In these cases, the outcome can be highly, if not
entirely, predictable. Coin tossing becomes physics rather
than a random event. It is the human element that makes the
process random in that each toss tends to be at a different
speed, sent to a different height, launched at a different angle
or caught in a different manner. Therefore, the possibility of
practising the task to reduce these differing elements can be
considered. If you try to toss the coin the same way each
time, you should be able to make the outcome significantly
different than 50:50.
A limitation of our study is the uncertainty about whether
the individual results are repeatable. However, we conclude
that the validity of using the toss of a coin to provide a random
50:50 outcome is thrown into doubt, both in medicine
and every day life.
Competing interests: None declared.
Contributors: Both authors contributed to the conception of the study
design, the acquisition and analysis of the data and writing and revising the
manuscript. Both authors approved the final version submitted for publication.
Matthew Clark accepts full responsibility for the work and the conduct
of the study, had access to the data and controlled the decision to publish.
Acknowledgements: The authors thank Hong Qian for her advice on the statistical
analysis.
uppermost side of the coin is known by the tosser (and caller).If this is not the case, the true 50:50 probability of the resultprevails. But if it is known, the side of the coin that starts offface up is more likely to end that way up because that sidespends more time facing up during the flight than does theopposite side. However, because of the large number of tossesrequired to detect this difference (shown to be 250 000tosses), this apparent difference is generally irrelevant.Coins have different raised profiles on each side. In theory,one side could be more weighted, thus making the coin tossunfair. This could be achieved by tampering with a coin. Withthe introduction of the Euro, Polish statisticians claimed thatthe 1 € coin (from Belgium), when spun on a surface, cameup heads more often than tails.3 This report resulted in atongue-in-cheek warning in the British press to teams playingagainst Belgium in the forthcoming soccer World Cup.4 Of250 spins, 56% came up heads. However, independent statisticalanalysis showed that random variation could producesuch scatter even with an unbiased coin given this number ofspins (acceptable range 43.8%–56.2%). Other research suggeststhat even if a grossly weighted coin is used (1 side lead,the other balsa wood), no significant bias shows up.3The usual method of tossing a coin should not be taken forgranted. Gary Kosnitzky, a Las Vegas magician adept in theart of coin magic, can reputedly manipulate coin tosses to givea predictable outcome by use of a method that gives the illusionof a spinning coin. When his coin “spins” in the air, it isnot rotating about its axis but is instead fluttering or wobblingand not turning over. This gives the illusion of a normallyspinning coin, yet gives a predictable result if caught in thepalm of the hand.5 Admittedly, this is probably an art morepractised by the gambler than the researcher, but it serves as areminder that an apparently fair situation can be manipulated.The most compelling finding that raises concerns about thevalidity of the coin toss come from the use of mechanical coinflippers, which can be made to impart exactly the same initialconditions for every toss, namely the starting position, velocityand force. In these cases, the outcome can be highly, if notentirely, predictable. Coin tossing becomes physics ratherthan a random event. It is the human element that makes theprocess random in that each toss tends to be at a differentspeed, sent to a different height, launched at a different angleor caught in a different manner. Therefore, the possibility ofpractising the task to reduce these differing elements can beconsidered. If you try to toss the coin the same way eachtime, you should be able to make the outcome significantlydifferent than 50:50.A limitation of our study is the uncertainty about whetherthe individual results are repeatable. However, we concludethat the validity of using the toss of a coin to provide a random50:50 outcome is thrown into doubt, both in medicineand every day life.Competing interests: None declared.Contributors: Both authors contributed to the conception of the studydesign, the acquisition and analysis of the data and writing and revising themanuscript. Both authors approved the final version submitted for publication.Matthew Clark accepts full responsibility for the work and the conductof the study, had access to the data and controlled the decision to publish.Acknowledgements: The authors thank Hong Qian for her advice on the statisticalanalysis.
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