17 June 2008

Age Curves for 2B Fielding

This will be a continuation of the recent Brian Roberts articles, but also the beginning of a new series. We often mention how certain positions age differently or utilize certain skills that are often age dependent. When it comes to fielding there are two main considerations when it comes to generating outs: the ability to field "cleanly" and range. Fielding cleanly or fielding efficiency is a skill that maximizes when the player has had experience at the MLB level. Aging will affect efficiency, but not to a great extent. Range on the other hand is heavily affected by aging, or that is what I would assume. As a player ages, he should experience decreased ability to cover the same territory or have his reaction time slow.

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 were considered.

Second Baseman Fielding Age Curves
The curves depicted to the right show the effect of age on fielding efficiency (orange) and range (black). It should be noted that each horizontal mark represents five plays. 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. Fielding efficiency maxes out around age 30 or 31. Range is maximized at age 23 or 24. 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.

The curves may be surprising. Well, they are surprising to me. I would expect them to have a much wider range in terms of efficiency and range. What may explain why I do not see this is that the tail end of the curve is being dictated by guys who can still play the position. Players who are not able to field are no longer at those position and would not be able to be included in the study.

Second base appears to be a position of considerable skill. It takes several years before fielding efficiency is optimized. Taking this data into consideration, defensive second basemen are hurt by free agency for the most part (or the organizations who sign them). After the renewal system and arbitration cycles take their turns, defensive minded second basemen hit free agency with their better days behind them. It is more likely that the dropoff is far more severe than depicted on these curves due to older 2B neutralizing the aging effect.

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