Friday, March 13, 2020
Statcast Lab: xERA
We’ve rolled out xERA, which simply takes xwOBA and puts it on the ERA scale. To show you that relationship, here’s xwOBA on the x-axis, and xERA on the y-axis. The idea is simply to make the stat more accessible. So, whichever scale you prefer, the OBP scale or the ERA scale, the same information is conveyed.
And if you square xwOBA, that curved line you see there will be a straight line. That’s the translation.
A few months ago, a poster in a fantasy baseball forum I participate in came up with a linear model for xERA from xWOBA (he simply plotted 3 years worth of wOBA vs. ERA with some minimum innings threshold).
I thought something more basic and fundamental was possible. In particular, I thought xERA should reflect the “expected” out rate, i.e. xBA should figure in the equation for xERA. My expression is:
The first term in square brackets is runs per plate appearance for the pitcher.
The second term in square brackets is an estimate of plate appearances per 9 innings. Batting outs per 9 is somewhere between 25.5 and 26. “bbfrac” is the pitcher’s actual walk rate (i.e. walks per plate appearance… I suppose one could include HB with walks). And xBA is expected batting average, of course.
Finally, I multiply by the league’s fraction of runs that are earned (0.924 in 2019).