It is a relatively simple concept. A pitcher’s kwERA is created by subtracting his BB% from his K%, and then adjusting to the league average ERA scale. By using just a few pieces of information, you have a metric that is good (or better) at projecting future ERA performance than more complicated metrics such as FIP, xFIP, and SIERA.
kwERA was originally formulated by Tom Tango, who is now the senior data architect for MLB Statcast. Jeff Zimmerman wrote this piece for The Hardball Times (RIP) explaining why kwERA should be used as the starting point for evaluating pitchers statistically. It shows a pitcher’s talent level in regards to the two outcomes that are most in the pitcher’s control; strikeouts and walks. In scouting terms, it shows us how well a pitcher limits damage by combining the dominance of his stuff with his control.
Below you will find the kwERA- of each minor league pitcher from 2019 with a minimum of 30 innings pitched at a particular level. kwERA- is created for each level that the player appeared at. So, for example, Tarik Skubal posted a kwERA- of 67 in the A+ Florida State League, then posted a kwERA- of 20 (!) in the AA Eastern League last year.
The kwERA- numbers are indexed so that 100 = an average performance for the level. Each integer below 100 shows that the pitcher’s performance was 1% better than average relative to the level he pitched at. So Tarik Skubal’s 20 kwERA- shows us that he was 1.8 times better than the average pitcher in the Eastern League last year.
|1||Tarik Skubal||Tigers (AA)||22||20|
|2||Spencer Howard||Phillies (A+)||22||31|
|3||Brendan McKay||Rays (AA)||23||34|
|4||Sam Delaplane||Mariners (A+)||24||36|
|5||Ryan Lawlor||Cubs (A+)||25||38|
|6||Scott Engler||Rangers (A)||22||39|
|7||Phil Bickford||Brewers (A+)||23||39|
|8||Joe Ryan||Rays (A+)||23||39|
|9||Kris Bubic||Royals (A)||21||43|
|10||Yerry De Los Santos||Pirates (A)||21||47|