Determinants of Major League Baseball Pitchers' Career Length

Purpose To investigate variables (injury, position, performance, and pitching volume) that affect the career longevity of Major League Baseball pitchers. Methods To be eligible, pitchers must have entered Major League Baseball between 1989 and 1992 without missing information for the variables on the website http://www.baseball-reference.com . The variables assessed were average innings pitched per year before and after age 25 years, earned run average, walks and hits divided by innings pitched, strikeout to walk ratio, pitching position, time on the disabled list, length of career, and starting and retirement age. We used analysis of variance to compare the differences between groups and a regression model to assess the relationship between variables before age 25 years and career length. Results Mean retirement age for the group was 31.74 (95% confidence interval 30.83-32.65) and mean career length was 10.97 (95% confidence interval, 10.02-11.92) years. Innings pitched after age 25 years increased slightly, but not significantly, from the number of innings pitched before age 25 years, 85.35 versus 74.25, P = .5063. Career earned run average was not significantly different after age 25 years compared with before age 25 years, 4.83 versus 5.58, respectively, P = .8834. Both strikeout to walk ratio, 1.55 to 1.77, P = .0022, and walks and hits divided by innings pitched, 1.63 to 1.50, P = .0339, improved significantly after age 25 years compared with before age 25 years. The position the player started and ended his career (starter or reliever) did not influence career length. Multiple regression analysis comparing the variables from before age 25 revealed only the number of innings pitched before age 25 were positively related to career length, R2 = 0.1408, P < .0001. All other variables analyzed before age 25 years were not significantly related to career length. Conclusions The only studied variable that had significant relationship, which was weak to low, with career length was innings pitched per year before age 25 years. All other variables analyzed before age 25 years were not significantly related to career length. Level of Evidence Level IV, case series.