Win Shares

Win shares is the name of the metric Bill James describes in his 2002 book Win Shares.

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.

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.