Nathaniel Boyden and James Carey, "From One-and-Done to Seasoned Veterans: A Demographic Analysis of Individual Career Length in Major League Soccer", Journal of Quantitative Analysis in Sports, 6(4): Article 5, 2010.

In this publication, the authors analyze data on over 1,000 Major League Soccer players in order to study the demographics of the league.  The findings of the study create an image of extremely short career spans, high attrition rates among first-year players, and elevated rates of turnover among foreign-born players.  At the same time, players who start consistently, advance from cameo to sustained appearances, and contribute significantly to team outcomes (especially by assisting on goals) have a greater chance to have long careers in MLS.

The motivation for this paper is perhaps best understood by knowing a little about the primary author.  Nathaniel Boyden will be most familiar to followers of UC-Santa Barbara soccer and perhaps to the most avid followers of the Seattle Sounders.  Boyden was highly regarded out of high school and played his college soccer at UCSB for four seasons.  His professional career was brief and was spent outside the top flight of American professional soccer — 13 appearances for the Seattle Sounders over two seasons (2006-07).  Boyden seems to have landed on his feet after his retirement from professional soccer, first working as a research analyst at one of the Max Planck institutes, and currently working toward his PhD in cognitive psychology at the University of Michigan.  So considering his background as a professional soccer player and his broader interests in the science of sport, Boyden would be well motivated to conduct the type of study of MLS players that has not been performed previously.

The data from the study were taken from MLS's official website, with player biographical data such as age and national origin from Wikipedia and other sources.  Over 1,000 players who were in the league between 1996-2007 were considered (those who played at least one minute in a league match), with over 1,100 spells  considered.  A spell is defined as a segment of time in which a player had signed with the league, so a player can have multiple spells in MLS.  It's worth mentioning that between 1996 and 2007 that the league had 10 teams, then 12, then reduced to 10, and later 12 and 13 teams.  The effects of league size is considered in this study.  Boyden and Carey adapt some analysis techniques from a study of Major League Baseball players to develop career expectancies of players at various years of service, and perform regression analysis to determine the effect of player performance, player characteristics, and league size on player career length.

The study reveals some expected, at times illuminating, and in other cases sobering results.  Here is a summary of them:

• The average MLS career is extremely short.  Boyden and Carey found that the average rookie player in MLS can expect to play for only two and a half seasons.  This expected career span is the shortest when compared to similar studies made in other sports, and perhaps the shortest in US professional sports leagues.  Attrition in the league is extremely high at 30%.  It should be mentioned that the authors did not perform further tracking of the players to determine which proportion of players were transferred to another top-flight league, moved to a lower-division league, or dropped out of professional football altogether.  That kind of study would take years on a sample size as large as the one considered here.
• If a MLS player makes it past the third year, chances are good for a relatively long MLS career.  Boyden and Carey show that the mortality rate spikes after the first season, but then levels out in subsequent seasons.  A player who becomes a regular starter in the league has a better chance of survival by year six, and as the physical skills decline in subsequent years (years 7+) the career expectancy decreases.  Nevertheless, over half of the players who enter the league in a given season are out in two years, and less than 20% of that incoming class are still in the league after five years.  
• How does one have a long MLS career?  Practice, and helping out.  The regression analysis showed that games played, minutes played, and assists are contributing factors to career expectancy in the league.  As one plays more matches, one gains experience in league play, which is important in order to become accustomed to the challenges and expectations of being a professional player and train accordingly.  Assisting on goals is of course hugely important; if a player is not contributing to the bottom line of a soccer team, he will be replaced by someone who can contribute — very quickly.
• Discipline and player position don't have much of a correlation with career expectancy.  There isn't a correlation between yellow or red cards and league departure risk in the player data, nor does player position make a huge difference.  Forwards/strikers do appear to have slightly shorter career expectancies.
• League expansion and contraction do have noticeable impacts on career length.  Boyden and Carey found that when the league has 12-13 teams, the risk of being out of a job in the league is much smaller compared to a ten-team league.  Of course, that will not be a surprise to anyone, but it is nice as a sanity check to see that expectation confirmed by the data.
• League departure risk is very high among foreign players.  Foreign-born players are found to be twice as likely to leave MLS as native-born players.  Another way to explain the results is that there is a high degree of turnover among the imported players — some don't pan out while others get better offers from home or retire.  It would be very useful to add age at the beginning of a player's spell as an additional regressor.  I think there were would be significant differences in risk between relatively young foreign players and those in the tail end of their playing careers (33 years of age or older).

The results presented in this journal article would be very useful to various people in the US professional soccer industry.  Sporting directors and player agents might use the actuarial tables for league players as a starting point in the valuation of players in their squad or portfolio, as well as the data on minutes played and assists to determine which players have a greater chance of an extended career in MLS.  Players union representatives could make note of the shorter career spans and the high attrition rates to make demands for improved compensation packages for their fellow players. League officials could use the data to develop strategies for recruiting and retaining foreign-based talent.  Finally players who are interested in a MLS career (as well as coaches and other guardians) can take a look at the career expectancy of incoming players and create a development plan that will enhance their chances of survival and at the same time ensure that they will have the appropriate skills for life after football.  In a league where the average salary is not yet six figures and is definitely not enough to ensure financial independence for life, such a plan is of the utmost necessity.