For much of the modern baseball era, a statistic called Earned Run Average was the pitching metric most widely used as an all-encompassing measurement of the overall performance of pitchers in a given season. In the last decade or so, other alternative statistics have come about to provide a stronger grasp of a pitcher’s ability to prevent runs.
ERA may not be as commonly used as it once was, but developing a core understanding of this stat can help lay the foundation for utilizing current and future run prevention metrics.
ERA explained
A pitcher’s most important job is to limit the number of baserunners and runs in a game in order to give the team the best possible chance to win. ERA was created to evaluate how well pitchers perform and measure how many runs they give up on average in a 9-inning game.
To calculate a pitcher’s ERA, take the number of earned runs and multiply it by nine, then divide by the number of innings pitched.
For example, consider a major league pitcher who allows 10 earned runs in 20 innings pitched. Ten Earned Runs multiplied by 9 innings is 90, divide that by the 20 innings pitched and you end up with an ERA of 4.50, about the league average ERA in the big leagues in 2021. Since it’s better for pitchers to give up fewer runs, the lower the ERA the better.
It’s important to note that ERA distinguishes between ‘Earned Runs’ and ‘Unearned Runs.’ Earned Runs are runs scored that do not come about by errors or passed balls. The rationale for this is that when evaluating a pitcher, we should not penalize him for baserunners not under his control.
However, ERA is less reliable in predicting future performance than other advanced metrics, such as Fielding Independent Pitching (FIP), xwOBA, and others. This is because ERA can be heavily influenced by factors that the pitcher has little control over, such as the skill of his team’s defense.
Additionally, ERA leaves out important contexts such as park factors or run-scoring environments. For example, a player who pitches in a pitcher’s park, like Oracle Park in San Francisco, might have an advantage over another player in Coors Field in Colorado, which has historically been known to be very hitter-friendly.
Summary
Compared to today’s tools, ERA has taken a backseat in analyzing pitcher performance at the highest levels. While it was the start of measuring run prevention in the recent past, many other metrics can more accurately measure a pitcher’s ability to prevent runs and predict future performance.
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