Shortstop Aging Curve

Yesterday, I posted about how age affects fielding performance for second basemen. It was found that:

Ages 27 to 29 are when fielding ability is greatest for second basemen as their efficiency rises and their range has not been greatly compromised.

Today, I will be focusing on shortstops. Again, the main fielding considerations for generating outs are the ability to field efficiently and range. Fielding cleanly or fielding efficiency is a skill that should maximize when the player accrues experience that assists in coordination, but then tapers off when age affects reaction time. Range typically has a shorter learning curve (i.e., first step), but more rapidly deteriorates due to age. Both of these general assumptions should apply to shortstops.

Revised Zone Rating (RZR) will be used as a surrogate for fielding efficiency. This metric assumes there is a given territory that a defender should be expected to cover. Of all the balls that pass through this zone, outs are recorded and compared to the number of chances. This is not ideal as RZR will be effected eventually by decreased range, but it should be rather representative because players are typically moved off positions if they are so unable to defend this standardized area.

Out of Zone (OOZ) Plays will be used to represent range. These plays are those that are made outside of the zone designated to the position. Again, there are potential issues. If a defender is playing next to a player who has great range then the number of OOZ plays he can accrue will probably be reduced. The resulting effect may not be great because several players seasons will be used to determine the aging curve line.

The data was collected from the Hardball Times fielding statistics. Fielding performance was recorded from 2004-2007. Out of Zone plays for each player was divided by the number of innings played and normalized over 162 9 inning games. Ages were then determined and applied to the seasons. Ages with less than three data points were removed from consideration. Only full time players with more than one season at SS during this time period were considered.

Shortstop Fielding Age Curves
The curves depicted to the right show the effect of age on fielding efficiency (orange) and range (black). Each horizontal mark represents five plays for both axises. The RZR line has been normalized for the average number of chances encountered at SS for a full 162 game 9 inning season. For instance, if a player moves from one horizontal line to the next over the course of two seasons then the player has improved or declined by five plays. A single play is worth about 0.6 runs, so each line represents 3 runs saved or lost.

Based on the age classes we have on hand (at least three data points had to be available for each age included), we were only able to include ages 22 to 32 on this graph. As opposed to the 2B curves, these have similar apexes, but differ with where they end up. Shortstop appears to take more skill and athletic ability to play effectively. This comes as no surprise. Also, range deteriorates much more quickly than efficiency, which agrees with the 2B study. Fielding lifespan is much shorter for a SS than a 2B though. Range for a SS seems to peak around 26/27 and fielding efficiency peaks 27/28.


Conclusion
A quick check on Google and I find that Tom Tango did something similar in February. His findings basically agree with my own even though we calculated these in different ways. His calculations predict a decline twice as rapidly as my own. For instance, we both find the same peak, but he finds a decrease of -35 plays from peak to age 32. I find it to be -18 plays. I am not sure which is more appropriate. Perhaps considering my findings and Tango's as a range would be a good idea. That range is worth about one win. Regardless, this trend seems more unmistakable.