From BR Bullpen
Game score is a metric created by Bill James to judge a pitcher's effectiveness in a single game. It uses a system of pluses and minuses to create a score. A score of 50 is considered an average outing. Scores of 0 or below or 100 or greater are extremely rare. The average game score for a starting pitcher at home is slightly higher than for the road starter, reflecting baseball's home field advantage. In practice, the difference comes from a better strikeout rate for home starters, which has made some authors speculate that the source may be some unconscious bias by home plate umpires, which give a few extra strikes per games to the home side.
Since 1957, the best game score was 116 recorded by Dean Chance in 1964. He threw 14 shutout innings giving up just three hits and striking out 12. The worst was a -21 recovered by Mike Oquist in 1998. Oquist gave up 14 earned runs in five innings.
From 1957 to 2007, there have been 40 games with a 100+ game score. Nolan Ryan had four such games. While game scores are closely correlated with pitching wins, this correlation breaks down at the very top end of the scale (scores of 90 or more), as many of these very high scores are achieved in games that go into extra innings, making it much harder for the starting pitcher to pick up the win.
The formula for Game Score from the baseball-reference main site is:
Start with 50 points. Add 1 point for each out recorded, (or 3 points per inning). Add 2 points for each inning completed after the 4th. Add 1 point for each strikeout. Subtract 2 points for each hit allowed. Subtract 4 points for each earned run allowed. Subtract 2 points for each unearned run allowed. Subtract 1 point for each walk.
 Further Reading
- Jeff Angus: "Does 'Game Score' Still Work in Today's High-Offense Game ?", in The Baseball Research Journal, SABR, Volume 39, Number 1 (Summer 2010), pp. 39-48.
- Peter Uelkes: "Games Scores: Matches, Correlations, and a Possible Umpire Bias", The Baseball Record Journal, SABR, Volume 41, Number 2 (Fall 2012), pp. 30-36.