SIERA quantifies a pitcher's performance by trying to eliminate factors the pitcher can't control by himself. But unlike a stat such as xFIP, SIERA considers balls in play and adjusts for the type of ball in play.
For example, if a pitcher has a high xFIP but has also induced a high proportion of grounders and pop-ups instead of line drives, his SIERA will be lower than his xFIP.
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)
Watch: Jake Arrieta combines strikeouts, grounders and pop-ups in a no-hitter that lowers his SIERA.
Why it's useful
A more advanced version of tERA, SIERA is meant to remove the volatility of ERA, while also allowing for the notion that balls in play are somewhat in the control of the pitcher. It also weaves together different aspects of pitching. So walks are a bit less harmful to pitchers who induce ground balls at a high rate, as ground-ball pitchers are more likely than fly-ball pitchers to have a walk erased by a subsequent double play.
SIERA is meant to be a better indicator of a pitcher's performance than ERA. Even though most fantasy leagues count ERA, SIERA can help assess a pitcher's talent and predict future success.