Comparing pitchers'ERA from the two major leagues, one should consider the role played by the designated hitter in the American league, surely a much tougher ninth batter to face than the weak, often incapable counterpart faced by the average NL pitcher.

So, considering last season's innings pitched and earned runs allowed by every pitcher in the big leagues, I came up with the idea of a "Designated hitter and pitcher neutral ERA" (DHPN), detracting from the average AL ERA all the RBIs scored by the players playing DH in their AL teams (849) and from the average NL ERA all the RBIs scored by the pitchers when they were at bat in the NL (259).

The result is an environment where the ninth batter is a totally inept player, with a batting average of .000 and no RBIs in the season: with this assumption in place, we can see the "DH effect" in play, and how it lowers the ERA of AL pitchers, who in a "neutral" environment would have a better ERA than their NL counterparts.