The job of a Major League Baseball player is to help his team win games, for the ultimate purpose of making the playoffs and winning the World Series. Since the early history of Major League Baseball, pitchers have been credited with Wins and Losses as official measures of the effectiveness of their pitching. Of course, Pitcher Wins are a fairly crude measure of how well a pitcher did his job, as wins are the product of the performance of the entire team – batters, baserunners, and fielders, in addition to pitchers.
While the implementation of Pitcher Wins as a measure of pitcher effectiveness is less than ideal, nevertheless the concept is perfectly sound. The ultimate measure of a player’s contribution – be he a pitcher, a hitter, a baserunner, or a fielder – is in how much he contributes to his team's wins.
Using play-by-play data, I have constructed a set of Player won-lost records that attempt to quantify the precise extent to which individual players contribute directly to wins and losses in Major League Baseball on the baseball field. Player won-lost records are described in a good bit of detail here.