A few weeks back, I investigated whether Tommy John surgery seemed to help fastball velocities as is often claimed. My finding was that there was no Tommy John effect; pitchers did not seem to experience any change in velocity post-surgery.
That said, in the original data the average and median differences in velocities were negative. Thinking this warranted a little more investigation, I performed a simple linear regression on the average velocities of each pitcher in my data set in an attempt to see if velocity prior to surgery helped predict velocity afterward.
Simple linear regression is used to describe the relationship between an independent and a dependent variable, here fastball velocity before Tommy John surgery (the independent variable) and velocity after Tommy John surgery (the dependent variable). Simple linear regression produces a regression line which is a straight line that has been fitted to the scatter plot displaying the independent and dependent variables (I'm sure you've seen this before in high school science class).
The formula for the straight line can then be used to analyze whether a relationship between the two variables actually exists or to see how successful the line is at explaining variation in the dependent variable. Regression is particularly interesting because it is often used for prediction; in our case, for predicting how fast Stephen Strasburg and Jordan Zimmermann will be throwing their respective fastballs when they return to the Nationals.
The simple linear regression I performed did turn out to be statistically significant and the R2 was about 0.75 which means about 75% of the error in simply using the average fastball velocity of all pitchers who received Tommy John surgery to predict fastball velocity of an individual pitcher after receiving Tommy John surgery was reduced by using the regression line equation. As for Strasburg and Zimmermann, the model predicts a one MPH reduction in Strasburg's fastball and a half-a-mile reduction in Zimmermann's.
While these results are a little on the dull side, they should still be encouraging for Nats fans: Zimmermann will likely have nearly all the speed he had before surgery and Strasburg should be expected to still throw harder than almost everyone when he returns 2012. Now all we need them to do is to never get Tommy John surgery ever again.