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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.

QERA(unscaled) =(2.69+K%*(-3.4)+BB%*3.88+GB%*(-0.66))^2

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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