Win Shares is a book about baseball written by Bill James and Jim Henzler. 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 formula involved in computing the final number of win shares accumulated.
Win Shares Digital Update, a companion volume of tables and statistics through the 2001 season.
Concept
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 player might accumulate around 20 win shares in a season. When a player exceeds 30 win shares, it is considered indicative of MVP-level performance, meaning the player is directly responsible for approximately 10 wins for their team. Achieving 40 or more win shares in a season is considered exceptional and represents a historic level of performance. For pitchers, win shares levels are typically lower, often closely aligning with their actual number of wins.
Win shares differs from other sabermetric player rating metrics such as Total player rating and VORP in that it is based on total team wins, not runs above replacement.
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.
Criticism of win shares
Players cannot be awarded "loss shares", or negative win shares, by definition. Some critics of the system argue that negative win shares are necessary. In defense of the system, proponents argue that very few players in a season would amass a negative total, if it were possible. However, critics argue, when one player does amass a negative total, he is zeroed out, thus diminishing other players' win-share totals. In an attempt to fix this error, some have developed a modified system in which negative win shares are indeed possible. The allocation of win shares 48% offense and 52% defense is justified by James in that pitchers typically receive less credit than hitters in win shares and would receive far too few win shares if they were divided evenly. 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 are 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).