Posted by Sean Forman on May 27, 2010
In order to keep up with the league leaders in WAR, we now show each league's top five on the front page every day just below the league standings. All of the Play Index tools are likewise updated daily.
How we compute Pitcher WAR
Pitcher WAR considers how many runs a pitcher allowed and then compares that to the number a replacement pitcher would allow.
To compute the amount from the replacement pitcher, we begin with the average runs allowed by pitchers facing the same set of teams for the same number of innings as the pitcher in question. We then adjust that number by the defense's runs saved for our pitcher (using Total Zone) per ball in play and our pitcher's team's park factor.
So now we have runs allowed by an average pitcher with our pitcher's opponents, defense, and park. We then multiply this by 1.22 or 1.18 (for the AL and NL respectively) to get replacement level for the AL and NL. This gives us Replacement runs, RR.
The replacement level for starters and relievers is different so we then decrease RR by 12.5% times the percent of relief innings thrown. The replacement level reliever has a much lower ERA than the replacement level starter.
We now get the pitcher's runs above replacement (RAR) as actual runs allowed - adjusted RR.
Next, we adjust for leverage and bullpen chaining by multiplying the RAR by (average leverage + 1)/2.
Finally, we adjust this into wins above replacement by dividing by a context-adjusted runs/win value (which is typically around 10).