THE BOOK--Playing The Percentages In Baseball

Monday, October 17, 2011

Starters and Relievers in the 9th Inning and Score Differential

There has been much discussion (and research) on the relative performances of starters and relievers in the 9th inning.  Preliminary results suggest that starters perform much better in the 9th inning, relative to their overall performance, than do relievers, even considering that they are facing the opposing lineup for the 4th time through the order.  The speculation is that these starters are having “on” days, their managers and coaches can recognize this, which is at least one reason why they are allowed to pitch (at least start) the 9th, and that their “onness” continued into the 9th inning.  (It is true that starters who are allowed to pitch the 9th inning – and 8th, 7th, etc. – have pitched exceptionally well in prior innings.)

While this is a reasonable assumption and certainly comports with conventional thinking, it is somewhat out of step with what we found in The Book – that early in a game success and failure does not have much predictive value.

I don’t necessarily have any particularly strong reason to disagree with this (new found – at least on my part) conclusion (that starters who are having exceptional games up to the 9th inning will continue to pitch at an above-average – for them - level), however, there are two things that I am uncomfortable with:  One, the small sample sizes of starting pitchers in the 9th – in my research I only looked at 2007-2010, and two, the fact that starters and relievers likely have different distributions of score differentials at home and on the road, and that these score differentials alone may considerably impact opponent wOBA (because their approaches may change and thus the wOBA weights are not correct).  This was brought up by Guy in one of the threads on this subject.

I was also troubled by one more thing:  When I looked at the 07-10 data for all starters and not just elite ones, so that I had a much large sample size, they did not pitch very well in the 8th, but pitched exceptionally well in the 9th.  You would expect that if starting pitchers who were “on” continued to be “on,” we would see this effect in the 8th (and 7th) as well as the 9th inning, although perhaps not to the same degree.  In fact, we should see a gradually increasing effect in every inning.  After all, a starter pitching the 8th, on the average, has pitched exceptionally well through the 7th inning, and presumably the manager and coaches allowed him to pitch the 8th not only because he has pitched well thus far, but because they deem that he is ”on” (and not just lucky).

So, one, I expanded my research to examine data from 1993 to 2010 (18 years), and two, I looked in more detail at how wOBA differs for relievers and starters in the 9th inning as a function of the score differential.

I included all relievers and starters in the analysis since I think that whatever happens in the 9th inning for all starters and relievers will also happen with elite starters and relievers (relative to their overall talent of course), and of course most relievers in the 9th ARE elite relievers and even starters in the 9th will tend to be the better starters.

Here is some of the data:

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