It is a measure to judge how effective a pitcher is. If it calculated by taking the total earned runs a pitcher has allowed and dividing by (total #of innings pitched/9). Giving you an average number...
Babe Ruth's career pitching record was 94 wins, 46 losses, 2.28 ERA.
David Ortiz doesn't have an ERA because he is not a pitcher. Only pitchers have Earned Run Averages.
Spud Chandler had a 1.64 ERA in 1943.
same as mlb but per w/e amount of innings they have