Baseball by the numbers: ERA+ better way to measure pitching

BY JOHN GROCHOWSKI

For the Sun-Times

Baseball numbers mean different things in different ballparks, and it’s part of the game that a pitcher whose home games are in Coors Field in Denver will allow more runs than one whose home games are in Dodger Stadium in Los Angeles. Pitching in Wrigley Field naturally will lead to higher ERAs than pitching in Busch Stadium in St. Louis, and U.S. Cellular Field is tougher on pitchers than Tropicana Field in St. Petersburg, Florida.

That’s why a column marked ERA+ is included on every pitcher’s page at Baseball-Reference.com. It takes ERA, compares it to league average, multiplies it by a ballpark factor and normalizes it to a 100-point scale so that an ERA+ of 100 indicates a league-average pitcher.

Take Dan Haren, who had a 4.02 ERA for the Dodgers last season, and Tyler Matzek, who was at 4.05 for the Rockies. The park effects lead to an ERA+ of 105 for Matzek and 87 for Haren. Give the environments in which they pitched, Matzek was a little better than average at preventing runs, and Haren was below average at doing so.

The basic calculation, as explained by Sean Forman at Baseball-Reference.com, is 100 x (2-ERA/league ERA). If the league-average ERA is 4.50 and a pitcher has a 3.50 ERA, then 3.50 is divided by 4.50, yielding 0.78. Subtract that from 2, and you get 1.22. Multiply that by 100, and a pitcher in a league-average park has a 122 ERA+.

Park factors basically come down to how many runs a team and its opponents score in a team’s home games compared with its road games. Baseball-Reference’s park factors for pitchers are weighted for every park in which the player pitches.

To go back to the Haren-Matzek example, Matzek had a park factor of 111.3, which means he pitched in parks that yielded 111 percent of league-average offense. Haren had a park factor of 95.5. So from a base ERA+, Matzek’s had to be multiplied by 1.113 and Haren’s by 0.955.

Pitchers on the same staff can have different park factors. It will affect the park factor if a pitcher has a greater percentage of appearances at home or if he pitches in tougher road parks than another. On the Cubs last season, Kyle Hendricks had a park factor of 105.5, while Jake Arrieta was at 104.8. On the White Sox, you could split hairs among Jose Quintana’s 101.8, Chris Sale’s 101.6 and John Danks’ 101.4.

Among Chicago’s starting pitchers with at least 100 innings in 2014, Sale was the leader with a 178 ERA+, followed by Hendricks (155), Arrieta (151), Jeff Samardzija (135 in the Cubs portion of his season), the Cubs’ Jason Hammel (128) and Quintana (116). Overall, Cubs pitchers faced a 103.9 park factor and Sox pitchers a 102 park factor, close enough that those pitchers rank in the same order in ERA as in ERA+.

There’s greater disparity among other ballparks, such as the 115.1 factor faced by the Rockies’ staff and the 96 faced by the Dodgers’ staff. Comparing pitchers by ERA+ helps level the playing field.