Each week, HighHeatStats.com will focus on one advanced statistic. This week: ERA+.
Throughout baseball history, changes in rules, changes in playing conditions and other factors have impacted the level of offensive output.
There was the American League's adoption of the designated hitter in 1973, the lowering of the pitcher's mound in 1969 and the end of the dead-ball era, circa 1920. Not to mention the 1893 modification of the distance between the pitcher's mound and home plate from 50 feet to 60 feet, 6 inches.
In addition to these and other factors affecting competition, you don't have to be a Colorado Rockies fan to know all ballparks were not created equally.
So it stands to reason comparing statistics from different eras and home ballparks doesn't exactly provide an apples-to-apples perspective. One stat that levels the playing field is ERA+, an invaluable measure that compares a pitcher's ERA to the league average and then makes an adjustment to account for park factors.
ERA+ helps us compare the run-prevention performance of a pitcher from the 1910s to one from the 1990s with a handy metric in which 100 represents league average.
Here are a few examples of how ERA+ can help us compare pitchers from different run-scoring environments:
The Los Angeles Dodgers' Chris Capuano posted a 3.72 ERA in 2012, good for an ERA+ of 102 (2% better than average). By contrast, in the hitter-friendly American League of 1996, the Baltimore Orioles' Mike Mussina had a raw ERA more than a run higher at 4.81, yet this translates to a slightly better 103 ERA+.
Pedro Martinez's career ERA of 2.93 is more than a run higher than all-time leader Ed Walsh's 1.82 mark. But since Walsh pitched in the dead-ball era and Martinez played during the hitter-friendly turn of the millennium, Martinez has a 154-to-145 career ERA+ advantage.
Even within a particular season, ERA+ can make it easier to compare. Last year, pitching in an offense-neutral park, the Washington Nationals' Gio Gonzalez recorded a 2.89 ERA, good for a 136 ERA+. In the higher-scoring American League, in a hitter's park, the Texas Rangers' Matt Harrison's 3.29 ERA was equal to Gonzalez's, also resulting in a 136 ERA+.
Dan McCloskey, HighHeatStats.com, an affiliate of USA TODAY Sports Digital Properties