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Introducing the PART System to DRS

October 28, 2019


As a regular reader of our work, you’re aware of how Sports Info Solutions tries to always be on the cutting edge when it comes to evaluating defense.
After a considerable amount of study, we’re releasing the latest update to our Defensive Runs Saved (DRS) metric.
Since 2013, our video tracking has allowed us to chart the pre-pitch infielder positioning. That data allows for the evaluation of defensive performance for individual fielders on shift plays and allows us to break the DRS system into PARTs. PART stands for Positioning, Air Balls, Range, and Throwing.
With this new system, we can isolate each of the individual skills for a player and separate those from positioning, which is largely controlled by a player’s team.
One of the other biggest benefits in this new system is the inclusion of shift plays.
Given the dramatic increases in shift usage in recent seasons, 
thousands of plays each year had been previously excluded from individual player ratings due to the current system's inability to separate the player and team’s positioning efforts from the player's range and throwing contributions on the play. Since the new system accomplishes just that, there's no longer any need to exclude these plays from individual player ratings.

How does this impact player value? The player who experienced the greatest increase in value in the new system was Athletics third baseman Matt Chapman, who went from 18 DRS to 34. This was a product of both crediting Chapman for his excellent fielding in defensive shifts and accounting for how the Athletics positioned Chapman. In the previous system, Chapman was being penalized for missing balls that he had no chance of reaching given where he was playing. In the PART system, Chapman’s positioning is taken out of his DRS and instead is assigned to the team.
Other players to receive DRS increases of 10 or more runs were Cardinals shortstop Paul DeJong, Rockies third baseman Nolan Arenado, Dodgers utility man Kiké Hernández (for his play at second base) and Cubs shortstop Javier Báez.
A comprehensive explanation of the methodology that produces the results will soon be available at and in the 2020 Bill James Handbook. They will be accessible at other publicly-available resources in the near-future as well.
When it comes to evaluating defense, we're not yet quite all of the way there, but we're getting closer. The introduction of the PART system provides a more accurate reflection of defensive performance.
Most Important Takeaways
  • Positioning is no longer factored into a player's Defensive Runs Saved Total
  • Components of player evaluation are now separated into Range, Throwing, Air Balls (how fielder fared on pop ups and fly balls)
  • This system allows for the evaluation of all infield plays, not just ones involving an unshifted defense
  • We have transitioned from evaluating "How often did a player make that play?" to "How often did a player make that play given where he was positioned?" with the PART system
  • The result is a more accurate overall depiction of defensive performance

COMMENTS (4 Comments, most recent shown first)

@steve161: Against the baseline of an average first baseman in a single season? Yes, but it's rare. Any of the eight positions can have a +30 season, but that's almost always a fluke. The scale should almost never go beyond +30, however.
8:36 AM Oct 29th
In principle this would fix a major hole in the DRS system. But the scaling issue remains. Does anybody outside of SIS really believe that a first baseman, no matter how excellent, can save a team 34 runs in a season?
6:53 AM Oct 29th
Supplementing what Charles said:

I'd be particularly interested to see how the figures and rankings 'with' and 'without' PART compare -- like, for given seasons.

Perhaps you weren't going to run any complete data that would allow such a comparison.
I hope very much that you will.

I think it's the onliest way that we (or you) will be able to see in any broad manner what kind of difference this addition makes.

Without that, we can't really tell how good or important or well-done this is.
11:03 PM Oct 28th
* How consistent is this from year-to-year?
* How well does it handle team defense in relation to the individual fielders?
* How well does it correlate to the traditional defensive statistics?​
10:16 PM Oct 28th
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