Win Shares is a statistic developed by Bill James. It is a sabermetric way of evaluating a players contributions to team wins. It takes hitting, pitching, and fielding into account. These statistics are then adjusted for the league, park and era. Win Shares measures the contributions of an individual player on the field relative to his peers. It also takes clutch hitting into account. Basically, it's a complex method with a simple goal: to compare baseball players.

A Win Share represents EXACTLY 1/3 of a team win. Thus, if a team has a total of 300 Win Shares on its roster, that team can be expected to win 100 games. Conversely, if a team wins 90 games, you can expect to find a total of 270 Win Shares on the roster.

Derek Jeter had 27 Win Shares in 1998 when the Yankees won 114 games. Jeter can be credited with 9 of those wins by James' Win Share system.

Calculating Win Shares is extremely complicated, but here's an explanation:

http://www.baseballgraphs.com/details.html#sharecalc
Hope that helps. I'm sure I've left a few things out, if anyone wants to add anything...

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