Reliability of Fastball Speeds for Pitchers Returning From DL

Recently, I published an article here at Fangraphs listing a few pitchers returning from the DL and their fastball speeds. One commenter, Eddie from NYC, noted:

Matusz has had exactly ONE start in 2011. Drawing any conclusion from one start is waay premature …. If his velocity stays at high 80s, yes, we should be concerned, but he’s only had one start.

I could not find via the interwebs how a pitcher’s fastball velocity in their first appearance back from the DL compares to their velocity the rest of the season. So I did a little research.

Some pitchers have made a living throwing slower, but generally the faster you throw the better you perform. As Mike Fast noted at the Hardball Times, starters expect their RA/9 innings to increase by ~0.25 for every 1 MPH decrease in fastball speed. The increase is ~0.40 RA/9 for relievers. Besides seeing an increase in ERA, the pitcher’s strike out rate decreases as their fastball speed goes down. So faster equals better for these purposes.

To get the needed data, I looked at pitchers that returned from the DL in 2010. I wanted to see how their fastball speed from a few appearances or starts (1 or 3) could help predict their fastball speed later in the season. I looked at pitchers that returned from the DL and made at least 6 starts or appearances over the rest of the season -or- before they went on the DL again (K-Dice, I am looking at you).

Originally, I planned on looking at all the pitchers returning from the DL, but the values effectively became unchanged after 25 samples (even though I did double that number). I will later finish collecting all the data later as I have 300 pitchers to go through and it takes 2-3 minutes each pitcher to collect the needed data. I try to have some kind of life outside of my basement.

Here are the results from the data I have collected so far:

Initial Game
Top Value (Initial Game Average – Season Average): 2.33 MPH
Average Speed: 0.15 MPH
Lowest Value: -1.75 MPH
StdDev: 0.80 MPH

3 Game Average
Highest (Initial 3 Game Average – Season Average): 1.04 MPH
Average Speed: -0.06 MPH
Lowest: -1.45 MPH
StdDev: 0.55 MPH

The overall average fastball speed is not much higher in the first start compared to the rest of the season. Pitchers don’t really see a bump up or down in speed as the season goes on compared to their first few games.

The spread in values are smaller after having 3 games worth of data than after 1 game. With only 1 game, the Standard Deviation (SD) is 0.80 MPH, but it is 0.55 MPH after 3 games. For those without knowledge on advanced statistics, 68% of all values will be within 1 SD and 95% of values will be within 2 SDs on statistics that are normally distributed.

For example, in his first game back Brian Matusz averaged 86.6 MPH. So there is a 68% chance that his fastball speed over the rest of the season would be between 86.6 +/- 0.8 MPH (85.8 to 87.4) or 95% of the time between 86.6 +/- 1.6 MPH. He has actually maintained the average of 86.6 MPH over the season so far. The effects from the ~3 MPH decrease from last season can be seen in a career high ERA of 5.60 which weirdly matches his current and career low K/9 of also 5.60.

As a general rule, one game of fastball speeds gives some useful information, but after 3 games a pitcher is generally throwing within 1 MPH of what they will average the rest of the season.