# A case of survivorship bias?

With the recent FIFA world cup, I decided to have some fun and determine which months produced world cup football players. Turned out, most footballers in the 2010 world cup were born in the first half of the year.

Someone pointed out, that children born in the first half of the year had a physical advantage over others and hence "survivorship bias" was involved in the equation. Is this an accurate observation? Can someone please explain why he says that?

Also, when trying to understand the concept, I found most examples revolved around the financial sector. Are they any other everyday life examples explaining it?

Thanks!

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The basic idea behind this is that football clubs have an age cut-off when determining teams. In the league my children participate in the age restrictions states that children born after July 31st are placed on the younger team. This means that two children that are effectively the same age can be playing with two different age groups. The child born July 31st will be playing on the older team and theoretically be the youngest and smallest on the team and in the league. The child born on August 1st will be the oldest and largest child in the league and will be able to benefit from that.

The survivorship bias comes because competitive leagues will select the best players for their teams. The best players in childhood are often the older players since they have additional time for their bodies to mature. This means that otherwise acceptable younger players are not selected simply because of their age. Since they are not given the same opportunities as the older kids, they don’t develop the same skills and eventually drop out of competitive soccer.

If the cut-off for competitive soccer in enough countries is January 1st, that would support the phenomena you see. A similar phenomena has been observed in several other sports including baseball and ice hockey.

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thank you user93! –  Preets Jul 19 '10 at 20:46
By "January 1st", I presume you mean the cut-off is an entire year and not 6 months period that applies to your children. –  Preets Jul 19 '10 at 20:48