Boston Red Sox: How does the rotation look now?

Then there’s FIP’s little brother, xFIP.

What do you get when you take a context neutral stat like FIP and decide you need it to be even more context-neutral You get xFIP. The adjustment made from FIP to xFIP is pretty simple. It takes the actual home run rate of the pitcher or pitchers and replaces it with the league average home run rate. The reason for this is that home runs per fly ball can be a bit fluky from year to year.

Plus, we start seeing those pesky variables that make the three true outcomes not quite so true popping up. That home run right down the line in right field at Fenway Park? It’s a pop out or maybe a double in Tampa — a lazy pop fly that scrapes the Green Monster? Easy out in Toronto. So enter xFIP to strip away even more of that stuff a pitcher has no control over. Like FIP, you want to compare it to other things. Namely, FIP and ERA. It actually might help answer why there’s a difference between them.

In this case, the Red Sox rank 8th in baseball in xFIP with a 3.77. Looks pretty similar to their FIP, so we can probably surmise that they haven’t had a massive amount of luck (one way or the other) on home runs so far. That backs up the guess we took above about them maybe seeing some improvement in their actual results (ERA).

And finally, there is SIERA.

There’s one more stat out there that tries to do what FIP and xFIP do, but it takes a slightly different path to get there. SIERA, or skill independent ERA, doesn’t ignore balls in play. But it does try to get into the nuts and bolts of results on balls in play. Here’s a link to the Fangraphs write up on the metric, and a quote from that page:

"“SIERA adds in complexity in an attempt to more accurately model what makes a pitcher successful. SIERA doesn’t ignore balls in play, but attempts to explain why certain pitchers are more successful at limiting hits and preventing runs. This is the strength of SIERA; while it is only slightly more predictive than xFIP, SIERA tells us more about the how and why of pitching.”"

Essentially, strikeouts are good, walks are bad, and higher ground ball rates lead to more outs on balls in play. So how do the Red Sox look by SIERA? They come in at 4th in baseball with a 3.62. 

Another indication that their 3.77 ERA might come down a bit. Of course, none of these metrics are suggesting a ton of variation between the quality of their outings and the results they are seeing. This is likely a top 5 or 6 rotation in the game, but they aren’t a threat to unseat the Astros at the top of the hill.