Thursday, December 6, 2012

Effect of age cut-offs on birth months of Div I college players

Following up on the birth month variations that are seen in the US Soccer Development Academy (USSDA) teams, I was curious to see if we see evidence of these age cut-off (Aug 1 club, Dec 31 ODP, Aug 31 HS) when we look at the distribution of birth months of players on Division 1 college soccer teams.  Participation on USSDA teams and top select (non-USSDA) teams might be expected to increase a high school player's chances of making a Div I college team, thus the deficit of quarter 4 births in the USSDA teams and May-June births on top select teams might carry over to college roster.

I took all Div 1 teams that made it to post season play ( and took birth months off their rosters.  I was able to get data on 562 players.  I excluded players who did not go to high school in the United States or Canada.  In retrospect, maybe I should have left off the Canadians, but there weren't all that many of them anyhow.  Then I went to the CDC and got the number of births by month for 1990-1994 to come up with expected numbers of players with each birth month.

The blue line shows the actual number of college players with the specified birth month and the red line shows the expected number given the births by month in 1990-1994.

Here are the differences (actual minus expected):

There is a deficit of 16.36 players with birth months in December. Interestingly there is a surplus of almost the exact number of players 16.34 with birth months in January.  Note this is based on a random sample of 562 players from the top Div I teams. If the pattern holds though-out the Div I rosters, then the total deficit of Dec players will be in the hundreds.

We also see that May-June birth months show a deficit while fall birth months have a surplus.  These patterns correspond to what we would expect given the age cut-off affecting US elite soccer players: Dec. 31 for the US Development teams and Aug 1 for select club teams.   Cut-offs for high school will also be important since many players are identified via performance on their high school teams.  School cut-offs vary regionally however.  The difference between percent summer and fall birth month may simply reflect a fall birth month advantage for D1 university students in general and have nothing to do with soccer age cut-offs.  It is well-known that the academic performance of kids born in summer is lower (on average) than that of those born in fall.  The effect is strongest in elementary school but can be seen all the way through high school.  See for example this literature review.

The 95% confidence intervals are about +13 and -13 for this so January and December fall outside of that while the summer months do not.  This suggests that I need to up the sample size a bit. 

I was able to find birth months data for 27 of the Div I teams that made it to post-season play.  When some birth dates were present for some players and others weren't, I collected data for the players I could.  I only took birth dates from the school roster webpages.  If a birth date was missing, but the players was on a US Dev team, I did not look up their birth date from the US Dev website since I assumed that would skew the data to January.  I only included players who went to high school in the US or Canada.
I got data for
Kentucky, Elon, Louisville, Marquette, UMBC, Boston College, Xavier, Winthrop, Mich State, UAB, U San Diego, UCLA, VCU, Cleaveland State, UConn, Creighton, Dickinson, Old Dominion, Syracuse, UNC, U Maryland, Indiana Univ, Wake Forest, Northeastern, Tulsa, Akron, UNM

The following schools did not have birth months on their websites:
Univ S Florida, Charlotte, St. Johns, Notre Dame, Air Force, UW, Drexel, Georgetown, Niagara, SMU, Cornell, Northwestern, Michigan, Lafayette

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