Friday, April 17, 2020

Looking at infield shifts

Even the more casual baseball fan can notice the seemingly peculiar way infielders align themselves in this day and age of baseball. As the camera follows the ball after it is put into play, we often see three defenders on the right side of second base, in the case of a left-handed hitter who tends to pull his ground balls. Popularized by the 2013 Pirates, as Travis Sawchik detailed in his book Big Data Baseball, the shift is an omnipresent phenomenon in baseball today, much to the chagrin of a certain segment of the baseball viewing public. With six seasons having passed since the Pirates, amongst others, took the league by storm with this new method of defending hitters, I thought it would be worth investigating the merits of the shift.

I compiled all plate appearances from 2017 through the 2019 season. MLB Advanced Media (which I will refer to as BAM henceforth) categorizes the infield alignment on a given pitch in three separate ways: standard, infield shift, or a strategic shift. Standard alignment is self-explanatory. An alignment labeled an infield shift is one where three infielders are on one side of second base. A strategic shift is one where there are not three defenders on one side of second base, but the infield is not in its standard alignment. The main impetus behind shifting the infield is two to try to turn more ground balls into outs than it would in a standard alignment. The following table represents the wOBA on ground balls in each infield alignment.

The more dramatic infield shift suppresses the effectiveness of ground balls by about 14.4%, while the strategic shift marginalizes the effectiveness to the tune of about 13.3%. This represents about 3.75 and 3.44 runs saved per 100 ground balls (without considering the base-out state), respectively. So, overall, the shift seems to be doing its job. However, we know the shift often is targeting left-handed sluggers with heavy pull tendencies. So, I filtered the data by including only left-handed hitters who pulled at least 40% of their batted balls:

Not much of a difference for the shift, it still performs worse relative to the performance of the standard infield alignment but these hitters on the whole perform worse on ground balls than league average. The strategic shift barely represents an added benefit to the standard alignment. Given these relatively surprising results, I tried to see if I can drill down the plate appearances where the shift is not adding more of a benefit to these left-handed hitters. Not all plate appearances are created equal; certain base out states yield better results. I separated each of these ground ball events by the base out state in which they occurred for this sample of 142 left-handed hitters. I also filtered out the strategic shifts to focus on the more drastic and typical alignment of three infielders to the right of second base. I then compared the results on ground balls for standard and shifted opposing infields.

This shows the various base-out states and the difference in wOBA between ground balls hit into standard infield alignments and shifts. Not much can be gleaned from this table on its own. Nevertheless, it can give you, the reader some insights into which types of situations the shift has worked against these pull-happy left-handed hitters. To check if the base-out state affected the shift’s potency, I compared the run expectancy of a base-out state to the corresponding effectiveness of the shift, which was the percent difference in wOBA for ground balls versus shifted and standard alignments.

A slight negative correlation (coefficient of determination of just -0.026), but almost definitely just noise. I cannot attribute the effectiveness of the infield shift for left-handed pull hitters to the base-out states where the shift was deployed. That is not to say the shift has not been effective for this specific type of hitter, one where you would think the shift is tailor-made for. My initial reaction is that the league has become adept at identifying the hitters they need to shift. So the obvious candidates do not perform worse than the entire league put together. Another possible explanation is I am not controlling for a variable that has a large effect on how well the shift swallows up ground balls. But that is an investigation for another day. One thing I think we can be fairly certain about is that the shift, six years later, is still an effective approach to defending ground balls.

All batted ball data via BAM, selected left-handed hitters via FanGraphs

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