RC (runs created): An estimator of how many runs a batter produces for his team, created by Bill James.

“Basic”: ((H + BB) * (1B + (2*2B) + (3*3B) + (4*HR))) / (AB + BB)

“Technical”: ((H + BB – CS + HBP – GDP) * ((1B + (2*2B) + (3*3B) + (4*HR) + (.26 * (BB – IBB + HBP)) + (.52 * (SH + SF + SB)))))/ (AB + BB + HBP + SF + SH)

Which formula and when: FanGraphs employs both formulas depending on what stats are available for individual seasons. Seasons prior to 1955 use the “Basic” formula and any season after and including 1955 uses the “Technical” formula.

Why you should care: Runs Created is a good estimator of how many runs a team should have scored in a given season. When applied to players, it is somewhat less accurate though still a useful estimator of a player’s actual production.

Variations: There are other run estimators that do a better job then Runs Created, yet one of the main advantage of Runs Created is that it’s extremely easy to calculate. Other run estimators include: Batting Runs, Base Runs, Extrapolated Runs, Estimated Runs Produced, Equivalent Runs.

Links and Resources:

Wikipedia: Runs Created
A Brief History of Run Estimation: Runs Created
How Runs are Really Created Part 1, Part 2, Part 3
The Book Wiki: Runs Created
The Book Wiki: Run Estimators

RE24 (runs above average by the 24 base/out states): RE24 is the difference in run expectancy (RE) between the start of the play and the end of the play. That difference is then credited/debited to the batter and the pitcher. Over the course of the season, each players’ RE24 for individual plays is added up to get his season total RE24.

Calculation Example: In game 4 of the 2007 World Series, the RE for the Red Sox to start the inning was .52. When Jacoby Ellsbury doubled off Aaron Cook in the very first at-bat in the game, the Red Sox were then expected to score 1.15 runs for the rest of the inning. The difference or RE24 was .63 runs. Ellsbury was credited +.63 runs and Aaron Cook credited with -.63 runs.

Why you should care: RE24 tells you how many runs a player contributed to his team. It’s similar to WPA (except in runs), but unlike WPA it does not take into account the inning or score of the game. Therefore, it is a more context neutral statistic. It does however take into account how many runners are on base and how many outs are left in the inning.

Variations: REW (run expectancy wins) is RE24 converted to wins.

Links and Resources:

Run Expectancy by Run Environment
The Book Wiki: Run Expectancy

WPA (win probability added): WPA is the difference in win expectancy (WE) between the start of the play and the end of the play. That difference is then credited/debited to the batter and the pitcher. Over the course of the season, each players’ WPA for individual plays is added up to get his season total WPA.

Calculation Example: In game 4 of the 2007 World Series, the WE for the Rockies started out at 50%. When Jacoby Ellsbury doubled off Aaron Cook in the very first at-bat in the game, the Rockies WE declined to 44.2%. The difference or WPA was .058 wins (5.8%). Ellsbury was credited +.058 wins and Aaron Cook credited with -.058 wins.

Why you should care: WPA takes into account the importance of each situation in the game. A walk off home run is going to be weighted more than a home run in a game that has already gotten out of hand. This makes it a great tool for determining how valuable a player was to his team’s win total.

When not to use it: WPA is more of a descriptive statistic and not that great of a predictive statistic. There are better statistics to use in raw player evaluations than WPA.

Links and Resources:

The Hardball Times: The One About Win Probability
The Book Wiki: Win Probability Added
Wikipedia: Win Probability Added
WPA is… WPA is not…

K/9 (strikeouts per 9 innings): The average of how many batters a pitcher strikes out per 9 innings pitched.

Calculated as: (SO * 9) / IP

Why you should care: K/9 is a perfectly suitable way to evaluate a player’s ability to strike batters out.

Current Baselines (2002-2007): The average K/9 for starting pitchers is 6.17 and 7.21 for relievers. For starting pitchers the top and bottom 20th percentile are a K/9 above 7.56 and below 4.89. Relievers top and bottom 20th percentiles are a K/9 above 8.94 and below 5.54.

Variations: Some people prefer to use strikeouts per batter faced (K% or K/G) to express a player’s ability to strike batters out. The difference is minimal and the argument for using K% is that K/9 excludes walked batters and K% does not, suggesting that K/9 may either overstate or understate a pitcher’s overall effectiveness (not pure strikeout ability).

Links and Resources:

Wikipedia: Strikeouts per 9 innings pitched
U.S.S. Mariner: Evaluating Pitcher Talent