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QuikERA (QERA) estimates what a pitcher's ERA should be based solely on his strikeout rate, walk rate, and GB/FB ratio.
These three components--K rate, BB rate, GB/FB--stabilize very quickly, and they have the strongest predictive relationship with a pitcher's ERA going forward. What's more, they are not very dependent on park effects, allowing us to make reasonable comparisons of pitchers across different teams.
Then, QERA is scaled to league average.
QERA(scaled) = QERA(unscaled) / League ERA * League QERA
Note that everything ends up expressed in terms of percentages: strikeouts per opponent plate appearance, walks per opponent plate appearance, and groundballs as a percentage of all balls hit into play. If, for example, Andy Pettitte has a 19.6% K rate, a 7.9% BB rate, and a 62.7% GB rate, he would have a QERA of 3.68. Note further that QERA is exponential, which is appropriate since run scoring is not linear.
Learn more in this article by Nate Silver.
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