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Win Shares is a book about baseball written by Bill James and Jim Henzler, published by STATS, Inc. in 2002. The book explains how to apply the concept of sabermetrics to assess the impact of player performance in a combination of several areas, including offensive, defensive, and pitching, to the overall performance of their team. The resulting "Win Share" also takes into account factors such as the era in which the player was active to allow easy comparisons between players from different eras. The book focuses primarily on the many formulae involved in computing the final number of win shares accumulated, as well as presenting lists of players ranked in various ways using the rating.
Win Shares Digital Update, a companion volume of tables and statistics through the 2001 season, was subsequently published in PDF form by STATS, Inc.
Win shares is the name of the metric developed by James in his book. It considers statistics for baseball players, in the context of their team and in a sabermetric way, and assigns a single number to each player for his contributions for the year. 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 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.
In baseball, all pitching, hitting and defensive contributions by the player are taken into account. Statistics are adjusted for park, league and era. 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.
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.
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.
Writing for ESPN, baseball author and columnist Rob Neyer called the book "groundbreaking". Glenn Guzzo echoed him in The New Ballgame: Baseball Statistics for the Casual Fan, calling it a "groundbreaking volume". Dave Studeman of Hardball Times observed that the book was well received by readers. Bill Felber, in The Book on the Book: An Inquiry Into Which Strategies in the Modern Game Actually Work, compares James' philosophy as to use of relievers in the book with those he endorsed as a Boston Red Sox executive.
In A Mathematician at the Ballpark: Odds and Probabilities for Baseball Fans, Professor Ken Ross describes the book as "erudite and interesting". The Oakland Tribune notes that it takes James more than 100 pages in the book to explain his formula. Leigh Grossman, in The Red Sox Fan Handbook: Everything You Need to Know to Be a Red Sox Fan Or to Marry One, called it "a book that statheads had been anticipating for years." The Concord Monitor noted that at a game "a few ultradedicated fans even sit in the back row with their Bill James books and calculators tabulating Mainers win shares". In Practicing Sabermetrics: Putting the Science of Baseball Statistics to Work, by Gabriel B. Costa, Michael R. Huber, and John T. Saccoman, the authors discuss how James provides both a short form method and a long form method for calculation in his book, and that the easier short form method appears to work well for years after 1920.