Dave Bush’s 2010 line, prior to this start:
18.2 IP, 14 H, 5 ER, 10 K, 6 BB 2 HR
.220 BABIP, 2.41 ERA, 4.46 FIP, 4.31 xFIP
Dave Bush’s career stats:
.290 BABIP, 4.64 ERA, 4.55 FIP, 4.37 xFIP
Dave Bush’s current 2010 line:
21.1 IP, 24 H, 12 ER, 12 K, 8 BB, 6 HR
.343 BABIP, 5.06 ERA
In sabermetrics, the concept of regression to the mean is very important – central, in fact. Wikipedia:
In statistics, regression toward the mean refers to the phenomenon that a variable that is extreme on its first measurement will tend to be closer to the centre of the distribution on a later measurement. To avoid making wrong inferences, the possibility of regression toward the mean must be considered when designing experiments and interpreting experimental, survey, and other empirical data in the physical, life, behavioral and social sciences.
In the context of baseball, and in particular this start, that means that we should expect a player to perform at his career levels as opposed to the levels established in a small sample, such as Dave Bush’s first start. Today was a great example of regression toward the mean – Bush’s stuff simply isn’t the kind of stuff that will play at a 2.41 ERA level, and this start has brought his 2010 numbers closer to his career mean.
At the same time, though, this start is no reason to freak out. Bush has had problems with the home run ball in the past, and occasionally will suffer games like today’s in which he can’t keep the ball on the ground. It’s disappointing that it had to come against the Cubs, but sometimes these things are unavoidable.
Dave Bush is not the 2.41 ERA pitcher that started the year. He’s not nearly as bad as today’s game suggests. What Dave Bush is hasn’t changed since last year – a slightly below average starting pitcher. Today simply wasn’t his day, but it should come as no surprise that he will no longer carry a sparlky sub-3.00 ERA into his starts.