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It considers statistics for baseball and basketball 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.
- 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[who?] 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).
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.
- James, Bill and Henzler, Jim. Win Shares, p. 2
- James, Bill and Henzler, Jim. Win Shares, p. 10
- Basketball-Reference.com "Calculating Win Shares" page
- Sortable Win Shares for 2004-2008 MLB - The Hardball Times
- "Fun With Win Shares", a blog featuring current win shares analysis and full 2003 win shares totals.
- Three articles criticizing win shares