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How Age Affects Infield Defense Using UZR Component Skills

In a previous article I looked at how the individual UZR components (Arm, Range, Errors) change as outfielders age. I have extended that study to include infield defense.

I looked at players who played the same infield position two years in a row and compared their UZR components, Range, Double Play, Errors and Total UZR (no range calculations for infielders) from the first to second year, weighting each pair by the number of innings played. If you want the complete explanation of all the calculations, please read the Complex Explanation in the outfielders articlep>




Here are the graphs for the four infield positions and a combination of all four. I am not sure how much stake I would put into the combination data since the hardest (shortstop) an easiest (first base) positions on the diamond are both included as infielders, but will still make it available for people examine. I assumed each player had a 0 (average) value for all attributes at age 22 and then plotted how his numbers would change as he aged:








As with the outfielders, the number of complete seasons worth of data are small for the early 20's and late 30's age brackets. Here is a graph of the number of complete match pair seasons for each age:




  • Total UZR decreased between 4 to 8 UZR for all positions across the 12 year  testperiod and, as with outfielders, a decrease in range was the leading contributor.

  • Age seems to have more of an effect on 2B and SS than 1B and 3B, mainly because Range is more important at SS and 2B.

  • Double Play rates remain constant over the years, not ever varying by more then 1 run.

  • Error rates also remain relatively constant, except for 2B error rates which start declining at age 31, and 3B error rates wich increase steadily over the whole 12 year period.

I hope that gives everyone a little more insight on how aging affects a player's ability to field.