ERA = (Earned Runs × 9) / Innings Pitched. Sub-3.50 is good, 2.xx is elite, 1.xx is rarefied.
An 'earned run' excludes runs scoring due to fielding errors or passed balls.
ERA is influenced by ballpark, defense, and luck. Modern analysis pairs it with FIP for a defense-independent view.
Hideo Nomo won the 1995 NL Rookie of the Year with a 2.54 ERA and 236 strikeouts.