Win Shares is a book (ISBN 1931584036) about baseball written by Bill James, published by STATS, Inc. in 2002. It takes a sabermetric approach to evaluating the contribution of individual players to their teams' overall performance, and focuses primarily on the many formulae involved in computing the final number, as well as presenting many lists of players ranked in various ways using the rating.
Win shares is also the name of the metric James describes in the book.
It considers statistics for players, in the context of their team, and assigns a single number to each player for his contributions for the year. All pitching, hitting and defensive contributions by the player are taken into account. Statistics are adjusted for park, league and era.
A win share represents one-third of a team win, by definition. If a team won 80 games in a season, then its players will share 240 win shares. Players cannot be awarded negative win shares, by definition, however some critics of the system believe that negative win shares are necessary. In defense of the system, very few players in a season would amass a negative total, if it were possible.
The formula itself is extremely complicated and contains a great deal of arbitrary numbers and educated guesses. It is a top-down approach which starts with the number of games a team won, and then attempts to assign credit to players, proportionally based on their statistics. Pitching and defense contributions receive 52% of the win shares and hitting contributions receive 48% of the win shares.
Hitting contributions are based on runs created. An attempt is made to then decide what amount of the pitching credit goes to pitchers and what amount goes to fielders. The pitching contributions are based on runs prevented, the pitchers' analogue to runs created. Fielding contributions are based on a number of educated guesses and a selection of traditional defensive statistics.
Win shares differs from other sabermetric player rating metrics such as Total player rating and VORP in that it is based on team wins, not runs.
One criticism of this metric is that players who play for teams that win more games than expected, based on the Pythagorean expectation, receive more win shares than players whose team wins fewer games than expected. Since a team exceeding or falling short of its Pythagorean expectation is generally acknowledged as chance, some believe that credit should not be assigned purely based on team wins. However, team wins is the bedrock of the system, whose purpose is to assign credit for what happened. Win shares are intended to represent player value (what they were responsible for) rather than player ability (what the player's true skill level is).
The Bill James Historical Baseball Abstract , 2001 edition, also written by James, uses win shares to evaluate the careers of many players, and to place them in contexts where they can be compared. The two books are effectively companions to one another.
In Major League Baseball, based on a 162-game schedule, a typical all-star might amass 20 win shares in a season. More than 30 win shares (i.e. the player is directly reponsible for 10 wins by his team) is indicative of MVP-level performance, and 40+ win shares represents an exceptional, historic season.
As of 2004, win shares are being used more and more in the sabermetric community but are not well known among casual baseball fans. Within the sabermetric community there is ongoing debate as to the details of the system.
See Also: Sabermetrics, Baseball statistics
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