| Abstract: |
Age effects in baseball are estimated in this paper using a nonlinear
fixed-effects regression. The sample consists of all players who have played
10 or more "full-time" years in the major leagues between 1921 and 2004.
Quadratic improvement is assumed up to a peak-performance age, which is
estimated, and then quadratic decline after that, where the two quadratics
need not be the same. Each player has his own constant term. The results show
that aging effects are larger for pitchers than for batters and larger for
baseball than for track and field, running, and swimming events and for chess.
There is some evidence that decline rates in baseball have decreased slightly
in the more recent period, but they are still generally larger than those for
the other events. There are 18 batters out of the sample of 441 whose
performances in the second half of their careers noticeably exceed what the
model predicts they should have been. All but 3 of these players played from
1990 on. The estimates from the fixed-effects regressions can also be used to
rank players. This ranking differs from the ranking using lifetime averages
because it adjusts for the different ages at which players played. It is in
effect an age-adjusted ranking. |