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# Statistics Phenomenon On The Pitch: Often Two Players With The Same Birthday At The World Cup

Date:
June 12, 2008
Source:
Universitaet Dortmund
Summary:
The German defender Philipp Lahm and the Portuguese midfield star Maniche were both born on November 11 -- and they were both playing in the game for the third place at the World Cup 2006. Anyway, in more than half of the games at the World Cup 2006 at least two persons on the field had the same birthday. That is what a young researcher found out within the scope of her bachelor thesis. And for the European Championship 2008 which has just started, one can expect a similar result. The reason is the so-called birthday paradox.
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The German defender Philipp Lahm and the Portuguese midfield star Maniche were both born on 11. November – and they were both playing in the game for the third place at the World Cup 2006. Anyway, in more than half of the games at the World Cup 2006 at least two persons on the field had the same birthday. That is what Yanina Lyesnyak found out within the scope of her bachelor thesis supervised by Prof. Walter Krämer. And for the European Championship 2008 which has just started, the graduate of the study program “data analysis and data management” prognosticates a similar result. The reason is the so-called birthday paradox.

It implies that the probability to find two persons with the same birthday within a group of 23 people chosen at random amounts to more than 50 percent. Yanina Lyesnyak discovered her interest in this phenomenon rather accidentally: “Two friends of mine are both called Katharina and share the same birthday – I always thought this to be fascinating”, explains the graduate. During her study she started to deal with this topic and wanted it to become the subject of her final thesis. Then she only needed an object of investigation. “I looked for something that involves 23 people to examine the birthday paradox. A soccer game already involves 22 players – and I just added the referee”, she reports.

And then she examined all 64 games of the Soccer World Cup 2006 in view of the birthday paradox. The result: in 53 percent of the games there were actually at least two people on the field who shared the same birthday. Sometimes even three; for example, in the game Argentina versus Serbia-Montenegro. And the game Netherlands versus Argentina was especially striking as there were three pairs with the same birthdays. In her analysis Yanina Lyesnyak just dealt with the initial team line-ups, the players taken off were not considered. “But the result would have been the same as the number of people being together at random would again be 23”, she explains.

To further substantiate her results, she also examined the Women’s Soccer World Cup 2007; and here the paradox was validated again. “To get an almost 100 percent hit rate one has to examine 66 persons. With 66 persons, picked at random, the probability that two of them share the same birthday is nearly 100 percent”, says Lyesnyak. She regards the season pattern of births as the reason for this phenomenon.

“Most of the children are born at the end of summer or at the beginning of spring. An the fewest in December and February”. This can at least be applied to Europe – and with 14 teams most of the teams at the World Cup came from Europe. It therefore has to be seen whether the birthday paradox will appear during more than 53 percent of all games of the European Championship in Austria and Switzerland.

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The above post is reprinted from materials provided by Universitaet Dortmund. Note: Materials may be edited for content and length.