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EVALUATING CHANGES IN, AND PREDICTORS OF, QUALITY STARTS AMONG PROFESSIONAL BASEBALL PITCHERS FROM 2015-2024

Abstract

In Major League Baseball (MLB), a quality start (QS) is awarded to a starting pitcher who pitches at least six innings while allowing no more than three earned runs. Quality starts are used to evaluate starting pitcher performance, often expressed as a percentage of total starts (QS%). PURPOSE: This study examined changes in QSs and QS% in MLB from 2015 to 2024, excluding 2020 due to the COVID-19 pandemic. It also aimed to identify season-level key performance indicators (KPIs) associated with QS%. METHODS: Using publicly available data, a season-level dataset including performance metrics (e.g., strikeout rate) and pitch-specific variables (e.g., fastball velocity) was created for the 2015-2024 regular seasons. General linear models were used to assess changes in QSs and QS% over time. A linear mixed-model, with individual pitchers as random intercepts, (R2 = 0.710, LRT = 10.5, p < .001) was used to examine the relationship between various season-level pitching statistics and QS%. RESULTS: Mean QSs decreased by 3.34 from 2015 to 2024 (t1096 = -5.03, p < .001). Similarly, mean QS% declined from 54.5% in 2015 to 43.9% in 2024 (t1096 = -5.17, p < .001). The linear mixed-model revealed that higher strikeout rate (F14,1010 = 65.0, β = 0.937, p < .001), lower hard contact rate (F14,966 = 43.7, β = -0.498, p < .001), lower pitch count per inning (F14,1075 = 532.54, β = -9.82, p < .001), higher ground ball rate (F14,745 = 18.25, β = -0.234, p < .001), lower pull contact rate (F14,1030 = 5.35, β = -0.183, p = .021), higher breaking ball velocity (F14,439 = 7.18, β = 0.233, p = .008), and induced vertical break (F14,443 = 4.84, β = -0.166, p = .028) were associated with increased QS%. CONCLUSION: While QSs declined from 2015 to 2019, a partial recovery was observed between 2021 and 2024. However, QSs and QS% in 2024 remained below 2015 levels, suggesting a long-term shift in starting pitcher performance. The significant associations between season-level pitching metrics and QS% suggest pitchers can improve the likelihood of a QS by increasing strikeouts, inducing more ground balls, limiting pull and hard contact, managing pitch counts efficiently, and enhancing breaking ball quality. Future research should investigate game-level data to further validate these findings or uncover alternative KPIs.

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