Earned Run Average has become the defacto mainstream metric when comparing pitchers . On the surface, it makes perfect sense, doesn't it? If a pitchers gives up X runs every 9 innings, and another pitcher gives up 2X runs per 9 innings, the former is most certainly the "better" pitcher!
Unfortunately, ERA is subject to a number of factors that are outside a pitcher's control. Let's go through how we break down each event a pitcher encounters(each at bat).
An at bat can result in a number of outcomes:
1) Strikeout: The batter accumulates 3 strikes and is out .
2) Walk: The batter accumulates 4 balls and is awarded 1st base keflex. (We'll also lump HBPs in here)
3) Ball in Play: The batter puts the ball in play .
4) Homerun: The batter hits a homerun, and awarded 4 bases without putting the ball into "play"
5) Other: The batter reaches via catcher's interference, etc. These "other" encounters are normally negligible throughout the season.
So, which of these can a pitcher actually control antabuse?
The first thing you have to consider is that whenever contact is made, the outcome of the event is suddenly out of the pitcher's control. No matter how great of a sinker Brandon Webb throws, Albert Pujols can still donkey kong it out of the park.
It's physically impossible for a pitch to suddenly "deaden" once contact is made, unless the pitcher has some magic flubber on the baseball. Once the ball makes contact with the bat, the pitcher been removed from the resultant outcome. The pitcher's contributions come from a combination of location, velocity, and movement.
What the Sabermetric community has discovered over many years of research is that the pitcher's skill in location, velocity, control, and movement is all he can do to influence an at bat. This results in him being able to affect:
- Strikeout Rate (K/9)
- Walk Rate (BB/9)
- Groundball Rate (GB%)
Now, it looks odd the GB% is there, being an outcome from a batted ball, right? What's we eventually determined is that pitchers with strong enough location and movement can keep the ball low in the zone for strikes, allowing them to create a situation where it is more difficult for the batter to square up on a ball. Although we've seen rare examples of Vlad golfing homeruns at his ankles, the ability for a batter to create solid contact on a ball low in the zone reduces the chance his contact turns into something besides a ground ball.
Naturally, the next step involves what happens to the ball when it is in play. A myriad of things can happen: Where are the fielders aligned? How good are they at covering ground and getting to balls to turn them into outs? Why should we judge a pitcher off a metric which is so dependent on 8 other fielders? As Troy outlined in his UZR study, the quality of the fielders behind the pitcher has a great affect on his ERA. Good fielders turn more balls in play into outs, and the difference between hits here and there can have a substantial affect on ERA.
With all of these other influences on ERA, what can we use to properly gauge future performance? If we stick with the idea of judging the pitcher solely on his contribution, we devise a metric that "represents" ERA based off the pitcher's skill.
Fielding Independent Pitching is a measure of everything which a pitcher is specifically responsible. The formula is (HR*13+(BB+HBP-IBB)*3-K*2)/IP, plus a league-specific factor (about 3.2) to round out the number to a value that "resembles" ERA, allowing us to quickly compare the two metrics. FIP was created by Tom Tango, author of "The Book"
The one addition to FIP that we need to account for is homeruns, and xFIP is my preferred metric to do this. Derek Carty of The Hardball Times did an excellent study on homeruns, and how and why a pitcher can prevent them.
To put it more simply, starting pitchers don't have any underlying ability to prevent home runs—the best they can do is prevent fly balls. If those fly balls are clearing the fence at too high a rate (or too low), we say that the pitcher has been unlucky (or lucky).
What does this leave us with? A few observations:
- ERA is not reliable when evaluating the future performance of a given pitcher.
- ERA is influenced by factors outside a pitcher's control, so critical analysis should be used when determining just how successful a pitcher is being, regardless of his ERA.
The next time we talk about ERA, we'll focus on what happens when contact is made, and introduce BABIP in depth.