Selection Bias in the NHL Draft

It has been a long standing practice in the National Hockey League to value slightly older players in the NHL draft.  Relative Age Effect occurs when people who are relatively older than the rest of their peers for their age group are more likely to succeed.  This phenomenon has been observed to reliably to occur in certain educational and athletic settings.  A group of psychology professors have discovered that NHL teams have been biased towards slightly older players in the NHL draft.  The research has shown that players that are born in the first three months of the year (relatively younger) are more likely to succeed in the NHL.  The study looked at twenty-seven years of data from the NHL and found that relatively younger players have a much longer career.  In the study they discovered that players who were born in between July and December accounted for 34% of the players drafted, but these players played in 42% of games, as well scored 44% of all the points.  On the other hand, players who were born from January to March accounted for 36% of the players drafted, but only accounted for only 25% of the points and only played in 28% of the games.  This discovery seems very odd to me.  It doesn't seem like which part of the year you were born in would have a substantial effect on your career in the NHL.  Also, this finding is in contrast with most other studies about Relative Age Effects, which state that relatively older individuals are more likely to succeed.  Another study showed that most of the top prospects (40%) in the Canadian youth hockey leagues were born in the first three months of the year, while only 15% were born in the later part of the year.  The study says they are not sure why this phenomenon had been occurring, just that it is an interesting finding and should merit further research and study.  I found this study to be very interesting, who knew that what part of the year you were born in would affect how well you preformed in a sport.

Sources:

http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0057753#abstract0

http://www.wired.com/playbook/2013/03/nhl-selection-bias/