How Do We Get the Numbers?

We are using a computer simulation to find the seasonal impact of individual players. This impact is called wins above average, or WAA.

To find a player's WAA, we assemble a team of league-average players by position for each year. If teams with these average players are simulated against each other, they should each win half the games and reproduce the league's average annual runs output. A selected player is then substituted for the average player at his position on one of the teams. Any change in that team's wins is taken as the player's unique impact, or WAA.

WAA can be below zero for a player whose impact is below average. It is zero for an average player. Note that WAA is not the same as the more familiar conventional measure WAR, or wins above replacement, which is calculated from estimates of the run potential of baseball events. WAR is also measured against a low baseline of replacement-player values, whereas WAA is measured against more challenging average-player values, and so is usually lower.

At the heart of the process is a simulation built from MLB data available since 1876. We thank Sean Lahman for the underlying data, without which this project could not have been done. The Lahman Database is available at SABR's Lahman Database page.

Ken Zweibel