Pitching Performance and Age By: Jaime Craig, Avery Heilbron, Kasey Kirschner, Luke Rector, Will Kunin Introduction April 13, 2016 Many of the oldest players and players with the most longevity of the game are pitchers. But does acquiring a pitcher who has been pitching for 20 years have any benefit? How does aging affect a pitcher s ability? Using the Lahman database, we analyzed pitchers from the years 1871-2014. We removed the pitchers younger than 19 years old and older than 40 years old. Because there are so few pitchers in these age frames, retaining the data from these pitchers would skew our results. There were so few pitchers younger than 19 years old and older than 40 years old that their stats were not representative of the rest of the pitchers. The histogram below (figure 1) shows the distribution of ages amongst the pitchers. Using ages as an independent variable, we looked at wild pitches, strike outs, and walks. We expected fewer wild pitches and walks as age increase. This is because as a pitcher ages, we expect that he becomes more accurate and precise. We expect the opposite effect of age on strike outs. We hypothesize that strike out rate will increase as age increases. A pitchers accuracy is expected to increase as he ages, causing more strikes and, as a result, more strike outs. Figure 1. Histogram of the ages of pitchers
Figure 2. Age vs Average Wild Pitches per Nine Innings Call: lm(formula = avg.wp ~ age, data = pitching.wp) Residuals: Min 1Q Median 3Q Max -0.025197-0.010433-0.002027 0.007257 0.038523 Coefficients: Estimate Std. Error t value Pr(> t ) (Intercept) 0.6088406 0.0193657 31.44 < 2e-16 *** age -0.0091124 0.0006443-14.14 3.43e-11 *** --- Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 Residual standard error: 0.01661 on 18 degrees of freedom Multiple R-squared: 0.9174, Adjusted R-squared: 0.9129 F-statistic: 200 on 1 and 18 DF, p-value: 3.433e-11 In the above graph, (figure 2) as age increases we see a decrease in wild pitches. This trend is a consistent negative linear relationship, and is statistically significant
(p<0.001). Pitchers at the age of 20 tend to have an average Wild Pitch/Nine Innings over 0.45. Meanwhile, pitchers older than 37 years old tend to have an average Wild Pitch/Nine Innings under 0.25. This result may indicate that older pitchers tend to be in more control. Younger pitchers throw faster and tend to overpower hitters with their fastball, thus tending to have more wild pitches. Older pitchers on the other hand have lower velocity pitches and thus have to focus on control to get batters out, leading to fewer wild pitches. Figure 3. Age vs Average Walks per Nine Innings Call: lm(formula = avg.bbr ~ age, data = pitching.bb) Residuals: Min 1Q Median 3Q Max -0.115411-0.040990-0.001246 0.046100 0.102429 Coefficients: Estimate Std. Error t value Pr(> t ) (Intercept) 5.776404 0.069386 83.25 <2e-16 *** age -0.066861 0.002308-28.96 <2e-16 *** ---
Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 Residual standard error: 0.05953 on 18 degrees of freedom Multiple R-squared: 0.979, Adjusted R-squared: 0.9778 F-statistic: 838.9 on 1 and 18 DF, p-value: < 2.2e-16 We analyzed the number of walks per nine innings by each age group. The results showed that walks had a statistically significant correlation with age (p<0.001), declining each year as players aged. At age 20, pitchers walked batters at an average of 4.5 times per nine innings, while at 30 this number decreased to approximately 3.7, dropping even further at age 38 to 3.25. Decreasing one year in age correlates with a decrease of 0.07 walks. This result was what we expected, as pitchers also showed a decline in wild pitches as they aged. This is most likely due to an increase in accuracy as a result of a decline in velocity, but general increase in overall pitching skill. Another reason for this decline could be attributed to the fewer number of pitchers lasting until their mid to late 30s. More inexperienced, and potentially lower-skilled pitchers are able to enter the league in their early 20s, while only the higher-skilled players are able to last into their 30s. This effect can be seen in figure 1 as the number of pitchers in the 30-40 year old age group is much lower than the number of pitchers in their mid to late twenties. Some of the most skilled pitchers have lasted until their late 40s, and the lack of sample size means that these players hold a large weight on the lack of walks, along with the lack of wild pitches. That should be noted, because our data does not necessarily follow the same pitchers from age 20 to age 38, but rather follows pitchers within these age ranges who still play in the MLB.
Figure 4. Age vs Average Strike Outs per Nine Innings Call: lm(formula = avg.sor ~ age, data = pitching.sor) Residuals: Min 1Q Median 3Q Max -0.56879-0.08721 0.01911 0.12925 0.21629 Coefficients: Estimate Std. Error t value Pr(> t ) (Intercept) 4.691836 0.220365 21.291 3.26e-14 *** age 0.014666 0.007331 2.001 0.0608. --- Signif. codes: 0 '***' 0.001 '**' 0.01 '*' 0.05 '.' 0.1 ' ' 1 Residual standard error: 0.1891 on 18 degrees of freedom Multiple R-squared: 0.1819, Adjusted R-squared: 0.1364 F-statistic: 4.002 on 1 and 18 DF, p-value: 0.06076 The graph above shows the average strike outs per nine innings (SOPNI) metric for different ages of pitchers. Although our SOPNI metric did not have a statistically
significant correlation with pitchers' age, the relationship between the two variables is still informative. In the graph above, you can see that SOPNI has two peaks at ages 27 and 36 with values equal to ~5.3, and an extreme low of ~4.4 at the youngest age we looked at, 20. This unusual result is relatively intuitive because a young pitcher should, in theory, get better over his years of play. Why there is a drop off in SOPNI values after 27 and then another spike at 36 is unclear. One possible reason for this increase in strike outs could be that these pitchers who have lasted into their later years are often some of the most skilled in the major leagues. That is to say, there is a lower sample size of pitchers in the 36-year-old range than in the 29-year- old range, and many of these pitchers have outperformed the average major leaguer for their entire career. This reduction in sample size, along with change in player skill within the sample size, could be a reason for the increase in strike outs in older pitchers. Conclusion Our results are consistent with our expectations of aging pitchers. Pitchers have become less wild with age. We also see that walks have gone down with age (figure 3). The graphs regarding wild pitches and walks show a negative linear relationship. This correlation can be attributed to the older pitchers being able to have more accurate pitches due to less power that has come with age. We have also noticed a somewhat parabolic curve with age in relation to strikeout rate that shows insignificance between the two variables. We have peaks at around age 27 and 36 with low points from the ages 30-35. It is interesting to see that there is no statistical significance between age and strike out rates (p=0.06076). Some of the data and trends can also be attributed to the fact that there are far less pitchers as age increases. We see a peak of pitchers around the age of 25 and this number steadily decreases. The sample size of pitchers has decreased which also may be a contributing factor to the trends we have noticed. The pitchers sampled in each range also change, and some of the decline in walks and wild pitches per nine innings could be attributed to the demographic change in these age brackets. Pitchers who are able to outlast Father Time are often some of the most skilled in the league, and will perform at a higher level in their late 30s than the average pitcher in their mid-20s. This effect should be taken into consideration, as the data collected could be changed in a future study to take this variation of age into account.