Win Shares can refer to a book by Bill James or the statistic explained in the book.
Win Shares is a book (ISBN 1-931584-03-6) 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.
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.
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 wins 80 games in a season, then its players will share 240 win shares. The formula for calculating win shares is complicated; it takes up pages 16-100 in the book. The formula contains many seemingly arbitrary constants and educated guesses. The general approach is to take the team's win shares (i.e., 3 times its number of wins), then divide them between offense and defense.
On a team with equal offensive and defensive prowess, hitters receive 48% of the win shares and those win shares are allocated among the hitters based on runs created. An estimation is then made to decide what amount of the defensive credit goes to pitchers and what amount goes to fielders. Pitching contributions typically receive 35 (or 36%) of the win shares, defensive contributions receive 17% (or 16%) of the win shares. The pitching contributions are allocated among the pitchers based on runs prevented, the pitchers' analogue to runs created. Fielding contributions are allocated among the fielders based on a number of assumptions and a selection of traditional defensive statistics.
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 responsible for 10 wins by his team) is indicative of MVP-level performance, and 40+ win shares represents an exceptional, historic season. For pitchers, Win Shares levels are typically lower--in fact, they often come close to mirroring actual wins.
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. The Hardball Times has developed its own Win Shares, as well as a number of derivative statistics, such as Win Shares Above Bench, Win Shares Percentage, Win Shares Above Average, and All Star Win Shares.