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The average American League starting pitcher has an ERA/RA of 4.37/4.76.
Red Sox starting pitchers have an ERA/RA of 4.64/4.96.
Yankees starting pitchers have an ERA/RA of 3.95/4.36.
On the other hand:
Red Sox RA in April, all pitchers: 5.72.
Red Sox pitchers since: 3.92
Yankees RA in April, all pitchers: 4.65
Yankees pitchers since: 3.84.
Seems to me that is darned close to even.