Thursday, November 29, 2012

It matters when you are born: the Matthew Effect

One of the questions I'll be looking at with the strength ratings data for multiple ages is the effect of age on strength of a team. Basically, 'what is a year worth?' and how much can a team potentially gain by being stacked towards kids who are older within the age group.  Note, it is not that coaches are looking at birth month when selecting players.  They are selecting the best players. Age and skill/physical prowess/speed etc. are highly correlated in kids, so the best players will tend to be the oldest in the age-bracket.

This is related to a well-known effect called the Matthew Effect, which you'll be familiar with if you have read the book Outliers. Basically, the existence of age-cutoffs means that kids born towards the beginning of a cutoff have an advantage and are preferentially selected for elite junior teams. These early advantages then translate to an over-abundance of players with birth months near the cutoffs on the most elite teams. So in our Aug 1 cut-off system, kids born in fall are at an advantage and summer kids are at a disadvantage. If the cut-off is Jan 1, then 1st quarter kids have an advantage. Here is a nice blog on the Matthew Effect: The Matthew Effect: Talent ID and sports science application.

Do we see this in soccer? Definitely. The US National Development program is on a Jan 1 cut-off. Here's some pie plots of the birth quarters of the players on the Sounders Academy U16, Crossfire U15/16, and US National U-15 men's teams. For Crossfire, there were 4 players listed as DP. I did not include them as the stats indicated that these players didn't play any games (so presumably DP means 'did not play'). As you can see, there is a clear advantage to being born near the Jan 1 cut-off. Notice the almost complete absence of quarter 4 boys. (The Crossfire team did have a couple qtr 4 boys but they were all "DP").

Does this translate to an over-abundance of players with birth years in the first quarter on the National Team (U-23)? Well, there is an over-abundance of those players on the US National Team.
The Crossfire and Sounders teams are U15/16, which means that some U15s are on the team. Let's look at the distribution of birth year on the teams and the birth months of the U15s:
My next post looks at the birth month effect across all the US Soccer Development Academy teams and shows that we see the '4th quarter' deficit across all team, but we also see a 'May-June' deficit.  The latter would be consistent with an effect of the Aug 1 cut-off on players before they enter the USSDA program.  Another post looks at the birth month effect and the 'December deficit' in the 2012 Olympic rosters and Division I US college soccer players.
Related work showing that even a 1-month average increase in the team's age has an effect on the team's performance:
Augst and Lames (2011) The relative age effect and success in German elite U-17 soccer teams

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