棒球得點圈打擊表現之因果推論與異質性分析：工具變數森林之應用

Abstract

本研究運用 2022 至 2024 年美國職棒大聯盟（MLB）常規賽的逐打席資料，探討在得點圈（RISP）情境下打擊表現是否存在因果效應。儘管球迷與媒體常把球員在得點圈的表現視為關鍵時刻發揮，但該效果在實證研究中始終存在爭議。為釐清此議題，本文採用 Instrumental Variable Forest（IV Forest）方法，兼顧內生性問題並估計異質性處理效應。實證結果顯示，RISP 對打擊表現有統計上顯著的負向影響，且該效果在年齡較輕、資歷較淺、近期打擊表現較好的打席中更為明顯。進一步分析發現，2023 年 MLB 實施限制防守轉位等新規則後，RISP 效果明顯趨近於零，顯示制度性改革不僅能改變場上的場上條件，也可能影響選手如何感受與因應高壓情境，進而影響其實際表現。整體而言，本文挑戰了「得點圈打擊表現僅為隨機誤差」的說法，並突顯球員特徵、比賽情境與制度改革交互作用的重要性。

This study investigates the causal effect of batting with runners in scoring position (RISP), using MLB plate appearance data from 2022 to 2024 season. While RISP performance is often associated with clutch ability by fans and media, its validity as a psychological or behavioral effect remains controversial in empirical research. To address this, we apply Instrumental Variable Forest (IV Forest), a machine learning method that accommodates treatment endogeneity and estimates heterogeneous treatment effects. Our findings show a statistically significant average negative effect of RISP on batting performance. Importantly, treatment effects vary across players, with younger, less experienced batters and those who have recently performed well showing greater negative impacts. We also discuss temporal shifts. In 2023, the average effect diminished to near zero following MLB rule changes, indicating that institutional reforms may reshape the psychological context in which high-leverage situations are perceived and responded to. Overall, our results challenge the notion that RISP performance is purely random, and instead emphasize the interaction of player traits, situational pressure, and institutional change.