Causal effect of the infield shift in the MLB

TL;DR

This study systematically analyzes the causal impact of the infield shift in MLB on offensive performance, finding it effectively reduces runs especially for left-handed batters, using multiple causal inference methods.

Contribution

It provides the first comprehensive causal analysis of the infield shift's effect on runs scored, informing policy changes in baseball.

Findings

Infield shift reduces runs scored, especially for left-handed batters.

All methods agree on the shift's effectiveness in preventing runs.

The 2023 rule change aims to limit the shift based on these findings.

Abstract

The infield shift has been increasingly used as a defensive strategy in baseball in recent years. Along with the upward trend in its usage, the notoriety of the shift has grown, as it is believed to be responsible for the recent decline in offence. In the 2023 season, Major League Baseball (MLB) implemented a rule change prohibiting the infield shift. However, there has been no systematic analysis of the effectiveness of infield shift to determine if it is a cause of the cooling in offence. We used publicly available data on MLB from 2015-2022 to evaluate the causal effect of the infield shift on the expected runs scored. We employed three methods for drawing causal conclusions from observational data -- nearest neighbour matching, inverse probability of treatment weighting, and instrumental variable analysis -- and evaluated the causal effect in subgroups defined by batter-handedness. The results of all methods showed the shift is effective at preventing runs, but primarily for left-handed batters.