Aaron Nola – A Pitch Value Comparison of Two Seasons


One of my favorite Phillies players is Aaron Nola. He has been the Phillies’ ace starter for several seasons, but he experienced a tough 2021 season, recording a 4.63 ERA in 32 starts. This is quite a contrast from his 2018 season where Nola had a sparkling 2.37 ERA, finishing third in the NL Cy Young award competition.

What went wrong with Nola in 2021? I thought it would be helpful to compare Nola’s 2018 and 2021 performances from a pitch value perspective. We’ll see some differences in pitch performance between the two seasons which might help Nola correct things for the upcoming 2022 season.

A Look at Aaron Nola’s Fangraphs Page

On Aaron Nola’s Fangraphs page, we see a variety of pitching stats by season which might help explain the differences between Nola’s excellent 2018 season and his mediocre 2021 season. Here are some things I observed by reading this page:

  • Actually Nola’s strikeout and walk rates were better in 2021 than in 2018. His strikeout and walk rates in 2021 were respectively 29.8% and 5.2%, compared with 2018 rates of 27.0% and 7.0%. Certainly he was striking out batters and minimizing his walks in 2021.
  • Following up this observation, I note that Nola’s 2021 FIP (fielding independent pitching based on SO, BB, and HR) was 3.37 which is much smaller than his 2021 ERA of 4.63. This suggests that Nola may have been hurt by the Phillies’ fielding in 2021.
  • Nola’s 2021 BABIP (batting average on balls in play) was .308 which was much larger than his 2018 BABIP of .251. This also suggests that Nola was not helped by his team’s defense.
  • Looking at the Statcast table, it seems that there were many more hard hit balls off of Nola in 2021 compared to 2018. The percentage of hard hit balls was 37.2% in 2021 compared to 30.8 in 2018, and there were 34 barrels in 2021 compared to 21 in 2018.

Pitch Value

In a post from January, I reviewed the concept of pitch value. Briefly, the value of a particular pitch is the change in runs value for balls passing through the old and new counts. I first collected the sum of pitch values for each of Aaron Nola’s starts for the 2018 and 2021 seasons. Here are boxplots of the total pitch values for starts for the two seasons. On average, Nola had a total pitch value of -0.62 on a start in 2021, compared with an average of -2.03 in 2018. Also we see that Nola had more extreme performances in 2021 than in 2018.

Pitch Values by Pitch Type

How do Nola pitch values vary by pitch type? Nola tends to throw four pitches, a four-seamer (FF), a curve ball (CU), a two-seamer or sinker (FT), and a change-up (CH). Here is a plot of the total pitch value for each pitch type for each of the two seasons. Note that Nola did worse on each pitch type in 2021 and he was especially worse on his curve balls.

Pitch Values by Count

How did his pitch values vary by count? Generally, one expects the pitch values to be negative for pitcher counts (like 0-2), positive for batter counts (like 3-1), and close to zero for neutral counts (like 1-1). Interestingly, comparing the 2021 and 2018 seasons, Nola did significantly worse in 2021 on 0-0, 1-2 and 3-1 counts.

What’s Happening on a 0-0 Count?

Let’s look further at what has happened on a 0-0 count for the two seasons. First, hitters were more likely to swing on the first pitch in 2021 (33.4%) compared to 2018 (30.6%). If they swing, they were less likely to make contact in 2021 (72.4%) compared to 2018 (75.8%). Most important, if they put the ball in play, their batting average on the first pitch was higher in 2021 (0.253) than in 2018 (0.196).

How did Nola do 10 units worse on pitch value in 2021 on a 0-0 count? Looking at the data, Nola did 2.3 units worse in 2021 on added strikes and the remainder appears to be due to the outcomes on balls put into play.

Pitch Values by Inning

How did his pitch values vary by inning for the two seasons? This graph is also interesting. In 2021, Nola did better than 2018 in the first inning, but significantly worse in the 2nd, 3rd and 5th innings. In 2018, Nola had low total pitch values in the 4th, 5th, and 6st innings which are key innings for a starter.

Locations of Curve Balls

I have demonstrated in a previous post that pitch values depend on location about the zone. So I was interested in comparing the locations of Nola’s pitches of a specific type for the two seasons. Here I graph the locations of his curve balls for the two seasons against left and right-handed hitters.

Looking carefully, it seems that there are differences in the proportion of curve balls thrown inside and outside of the zone. Define a pitch where pitch_x < -1 as “left” and a pitch where pitch_x > 1 as “right”. To left-handed hitters, the proportion of inside pitches (“right”) increased from 0.091 (2018) to 0.137 (2021). In contrast, the proportion of inside pitches (“left”) to right-handed hitters dropped from 0.036 (2018) to 0.013 (2021). It was rare for Nola to locate an inside pitch to right-handers. (Nola would have to aim his curve ball at the right-handed batter to get an inside pitch out of the zone.)

##   Season stand Prob_L Prob_R
##   <chr>  <chr>  <dbl>  <dbl>
## 1 2018   L      0.117  0.091
## 2 2018   R      0.036  0.263
## 3 2021   L      0.076  0.137
## 4 2021   R      0.013  0.26

Locations of Four Seamers

These inside/outside patterns were more obvious for Nola’s four-seamers for the two seasons. Looking at the FF thrown to right-handed batters (right panels), note how the locations have shifted towards the outside.

Defining inside and outside the same way, we see below that the proportion of outside pitches to right-handers increased from 0.105 (2018) to 0.205 (2021). In contrast, the proportion of inside pitches to righties decreased from 0.137 (2018) to 0.045 (2021). There was a clear strategy by Nola in 2021 to not throw fastballs inside to right-handers. In contrast, the proportion of outside pitches to left-handers decreased from 0.182 (2018) to 0.117 (2021). This is surprising — wouldn’t one want left-handers to chase at fastballs out of the zone?

##   Season stand Prob_L Prob_R
##   <chr>  <chr>  <dbl>  <dbl>
## 1 2018   L      0.182  0.11 
## 2 2018   R      0.137  0.105
## 3 2021   L      0.117  0.086
## 4 2021   R      0.045  0.205

Some Takeaways

  • Nola 2021 struggles. From the perspective of pitch value, Nola had a tough 2021 season compared to his stellar 2018 season.
  • Where did Nola struggle? I think one should first identify areas where Aaron Nola struggled in the 2021 seasons. We see some interesting count and inning effects — for example, he struggled on 0-0 counts and in the 2nd, 3rd, and 5th innings. The 2nd and 3rd inning issues are disappointing, since one tends to pitch to weaker hitters in the bottom of the bat order in those innings.
  • Pitching patterns. As an aside, we picked up some interesting differences in pitch location. For example, Nola appeared reluctant to throw his four-seamer inside to right-handed hitters. I wonder why.
  • Further work? This is an initial exploration of Nola’s performance from the perspective of pitch value. For further work, I would look at qualities of his pitches such as speed, break, and spin, focusing on differences between the two seasons.
  • Prediction? As a Phillies fan, I am optimistic that Nola and his team will sort out some of these issues and perform better in the 2022 season.
  • Interested in pitch value? Earlier this year, I had two posts on pitch values. The first post graphically shows how pitch value for specific pitch types varies across the zone. The second post show the important role of the count in pitch values.