Regression and Pineiro
Recently there has been some discussion about estimating a player’s true talent level. The idea is that a player's true talent, and how we should expect him to perform going forward, is not the player’s current level of production. Rather it is a weighted average of his current year and past production (with more recent production weighted more heavily) and then this average is regressed to league average, with the amount of regression depending on how many plate appearances (for batters) or batters faced (for pitchers) or inning played (for fielders). The details of how to do the weighting and to which population’s mean you regress are important and discussed at the Book Blog and THT.
I wanted to look at an example of a player whose current year production is far out of line from a long career of established production. Joel Pineiro leads all starters in ground ball rate, at 60.4% ground balls per ball in play. Since 2002 his GB rate ranged between 44% and 48%. In addition, Pineiro leads all starters league in BB per batter faced at 2.6%, again far out of his previous range of 5.4% to 8.5%. This is a rather huge shift in his numbers.
Here are his five pitches.
The movement on these pitches is fairly standard. It is important to note his two-seam fastball ‘sinks’ compared to his four-seam fastball. Here is the breakdown of his pitch usage over the past three years, those covered by PITCHf/x.
| | 2007 | 2008 | 2009 |
| Four-Seam Fastball | 0.54 | 0.36 | 0.11 |
| Two-Seam Fastball | 0.03 | 0.23 | 0.60 |
| Slider | 0.16 | 0.20 | 0.11 |
| Curve | 0.16 | 0.09 | 0.09 |
| Changeup | 0.11 | 0.12 | 0.09 |
His two-seam fastball is hit on ground just under 68% of the time it is put in play, so his increased usage of that explains the jump in grounders. He gets his fastballs in the zone about 54% of the time while his breaking and off speed pitches are in the zone under 50% of the time (47% for his change, 42% for his curve and 49% for his slider). Finally batters swing at and make more contact with his fastballs than his off-speed and breaking pitches. As a result he has many fewer walks and strikeouts (he has struck out just 10% of batters the lowest rate in his career).
I think this is an interesting example in which the PITCHf/x data partially explains a recent abrupt change in numbers. Obviously we do not expect Pineiro to continue to walk under 3% of batters faced and get over 60% of his balls in play on the ground. An estimate of true talent and expectation going forward must include some weighting of past performance and regression to the mean. But I think the PITCHf/x data, just like scouting data, can be used to adjust the weighting, maybe weight this year even more heavily if we expect him to use this pitch break down going forward, or regress to different pool, one with this breakdown of pitches, to get a better estimate of his true talent going forward.