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What Average Leverage Index Revealed About the 2021 Diamondbacks

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By looking at this season, we observe ways to add wins next season.

Luke Weaver pitched against Padres on 1 Sept. It was the 7th lowest aLI win.
Luke Weaver pitched against Padres on 1 Sept. It was the 7th lowest aLI win.
Photo by Norm Hall/Getty Images

Leverage index is often used to describe the potential impact of a specific at-bat, and whether a specific player had better results in high impact situations. When the leverage index is high, an at-bat can change the team’s win probability by a huge amount. When it is very low, the outcome of the game leaves almost no room for doubt.

Instead of leverage index, this article focuses on average leverage index for each complete game. My view is that in general, when leverage was low, the game was decided in the early innings; when leverage was high the game was decided in the late innings.

For the 2021 Diamondbacks season, let’s look at each game’s average leverage index from the Diamondbacks’ perspective. For each game played, Baseball Reference provided two measures:

  • aLI for Diamondback batters (which I will abbreviate as aLI-b).
  • aLI for Diamondback pitchers (which I will abbreviate as aLI-p).

Comparing Diamondback batters to the Dodgers.

Let’s look at aLI-b for three teams, the Diamondbacks, the Rockies, and the Dodgers. Remarkably, the following graph shows that when aLI-b for a game is high, the Diamondbacks and Rockies are as good as the Dodgers as far as winning percentage.

2021 season. Data from Baseball Reference.

Diamondback pitching won half of high leverage games.

Let’s switch from batting leverage to pitching leverage. In the context of a season with a winning percentage of 32.1%, the following graph shows the Diamondbacks winning percentage for 5 ranges of aLI-p.

The graph shows that for games in the lowest range of aLI-p, the Diamondbacks winning percentage was 20.5%, which was much lower than the season winning percentage.

The graph shows that for games in the highest range of aLI-p, the Diamondbacks won 50.0% of games, much higher than the season’s winning percentage. If more games had a higher aLI-p, perhaps the Diamondbacks’ season winning percentage would approach 50%, which would be a big improvement.

2021 Diamondbacks. Data from Baseball Reference.

Losses that were very winnable.

This season, the Diamondbacks lost 31 games by one run. The aLI-p was above average for 21 of those games - indicating they were very winnable by Diamondback pitchers. Strengthening that conclusion was that of the 21 very winnable games, the following chart shows that 9 of those games were mainly lost due to starting pitching and 7 of those games were mainly lost due to bullpen pitching.

The following chart shows main reason that each game was lost, as well as providing the aLI-p.

One-run losses for the 2021 Diamondbacks. Data from Baseball Reference.

Nevertheless, better offense could make an impact. In four games lack of offense was identified as the main reason for the loss. Another supporting reason is that in 9 games when starting pitching allowed too many runs (and was identified as the main reason for the loss), the games were still winnable. Perhaps the best approach is to score more runs early in the game (because offense late in the game may be less likely).

It’s possible that batting in high leverage games is a learned skill. Looking at individual Diamondback batters in nine games with the highest aLI-b, two batters stood out. Pavin Smith had three games with a positive batting Win Probability Added (WPAs of .185, .509, and .033). Christian Walker had three games with a positive batting WPA (.112, .128, and .183). Perhaps much could be learned from their success.

Games that Diamondbacks won easily.

Next season, wouldn’t it be great if the Diamondbacks locked up wins early in many games? Lets look at the 10 games with the lowest aLI-p that the Diamondbacks won.

  • In those 10 games, after the first 3 innings were completed, Diamondback batters had scored an average of 5.3 runs, which is higher than the season average of 1.3 runs.
  • In those 10 games, Diamondbacks pitchers pitched to an average of 7.1 batters per game beyond the minimum of 3 batters per inning, which was lower than the season average of 12.4 batters per game. The Diamondbacks’ season average was slightly higher than the league average of 11.1 batters per game.

Which Diamondback pitchers started those 10 games? Luke Weaver pitched in three games, Bumgarner and Kelly each pitched in two games, Gallen, Gilbert, and Peacock each pitched in one game. Interestingly, this aligned very well with the 5 predicted opening day starters in this AZ Snake Pit article. The big difference was that the article predicted Taylor Widener instead of Merrill Kelly.

Let’s look at Taylor Widener to see why none of his games made the top-10 list. On 23 May, he injured a muscle and left the game after 1.2 innings - that start will be excluded leaving 12 games.

In the first 3 innings of his 12 starts (36 innings), he allowed 11 runs (excellent) while the Diamondbacks scored 12 runs (six games with zero runs scored and six games with an average of 2 runs scored). The absence of games with Diamondback batters scoring 4 or 5 runs in the first three innings meant that his games were not low leverage.

Before proceeding, the following table shows the Diamondback wins with the lowest aLI-p.

2021 season. Dback wins with lowest aLI-p. Data from Baseball Reference.

The Diamondbacks who were most often at the plate when runs scored were highlighted in different colors. In these 10 games, David Peralta was an offensive dynamo, especially in the first three innings.

The table shows four wins against the Padres. Perhaps the Diamondbacks match-up well against the Padres. That was indeed the case because:

  • Diamondbacks’ runs-scored against-the-Padres divided by runs-allowed against-the-Padres was the 4th highest of the 19 teams the Diamondbacks played. For this measure against the Padres, the Diamondbacks were better than against any other team in the NL West.
  • Diamondbacks’ winning percent against the Padres was the 7th highest of the 19 teams that played against the Diamondbacks.

Summary.

In games with high batting leverage, the Diamondbacks are as good as the Dodgers as far as winning percentage. Nevertheless, the best batting approach could be to score runs in the first three innings. David Peralta is an offensive dynamo in those innings. Furthermore, in the nine games with highest batting leverage, the success of Pavin Smith and Christian Walker stood out.

In games with highest pitching leverage, the Diamondbacks’ winning percentage was 50%, which was better than the season’s average winning percentage of 32.1%. 21 of the Diamondbacks’ one-run losses were very winnable because they had above average pitching leverage. More specifically, 9 games were lost mainly because starting pitching and 7 games were lost mainly because bullpen pitching.

The ten easiest Diamondback wins had low pitching leverage. They had two significant characteristics:

  • The Diamondbacks scored more runs in the first three innings (5.3 vs 1.3) than the season average.
  • The Diamondbacks pitched to less batters beyond the minimum (7.1 vs 12.4) than the season average.

Of the 19 teams the Diamondbacks played in 2021, the Diamondbacks matched up well against the Padres. Two reasons are:

  • Their winning percentage against the Padres was the 7th highest.
  • Their runs scores divided by runs allowed against the Padres was the 4th highest.