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Skill-Interactive Earned Run Average estimates ERA through walk rate, strikeout rate and ground ball rate, eliminating the effects of park, defense and luck.
SIERA accounts for how run prevention improves as ground ball rate increases and declines as more whiffs are accrued, while grounders are of more materiality for those who allow a surplus of runners. The formula for SIERA is:
SIERA = 6.145 - 16.986*(SO/PA) + 11.434*(BB/PA) - 1.858*((GB-FB-PU)/PA) + 7.653*((SO/PA)^2) +/- 6.664*(((GB-FB-PU)/PA)^2) + 10.130*(SO/PA)*((GB-FB-PU)/PA) - 5.195*(BB/PA)*((GB-FB-PU)/PA)
where the +/- term is a negative sign when (GB-FB-PU)/PA is positive and vice versa.
This was explained in more detail in the following series: part one, part two, part three, part four, and part five.
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