The Turning Point - Limitations of Fielding Percentage
For decades, defensive evaluation in NPB centered on fielding percentage, which measures the probability of not committing an error. However, this metric has a fundamental flaw: if a fielder cannot reach a batted ball, no error is recorded, effectively penalizing players with greater range. For example, in the 2018 Central League shortstop fielding percentage rankings, players with limited range and fewer chances ranked higher, while wide-ranging defenders like Hiroshima's Ryosuke Kikuchi (then at second base) and Kosuke Tanaka did not stand out. To address this contradiction, zone-based defensive metrics such as UZR (Ultimate Zone Rating) and DRS (Defensive Runs Saved) began appearing in Japanese baseball media and data sites from the late 2010s. The pivotal moment came around 2015 when DELTA Inc. started calculating and publishing UZR for all NPB players, marking the beginning of Japan's defensive metrics revolution.
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How UZR and DRS Work
UZR divides the field into granular zones and evaluates how many more (or fewer) outs a fielder converts compared to the league-average fielder in each zone, expressed in runs saved. For instance, a player with a UZR of +15.0 prevented approximately 15 runs more than an average fielder over a full season. DRS classifies batted balls more precisely by speed, angle, and direction, factoring in each play's difficulty level. In MLB, Baseball Info Solutions calculates DRS, while in NPB, DELTA Inc. and Data Stadium Inc. compute UZR using proprietary zone data. Both metrics are superior to fielding percentage in comprehensively evaluating range, double-play ability, and throwing accuracy. However, mid-season data with small sample sizes is unreliable, and at least one full season of data is considered necessary. Additionally, catcher framing and pitcher defense are evaluated using separate specialized metrics.
Re-evaluating NPB's Elite Defenders with Metrics
The introduction of advanced defensive metrics has significantly changed how NPB's elite defenders are evaluated. Seibu Lions' Sosuke Genda has maintained league-leading UZR as a shortstop since joining in 2017, recording a UZR of +18.3 in 2019. From a sabermetric perspective, this equates to approximately 2 wins above replacement from defense alone. Hiroshima's Ryosuke Kikuchi won six consecutive Golden Glove Awards as a second baseman from 2014 to 2019, and his UZR exceeded +10 each year during that period, making him a rare case where subjective evaluation and objective metrics aligned. However, since Golden Glove selections are based on reporter voting, cases where UZR leaders miss the award are not uncommon. In 2022, the Pacific League first baseman with the highest UZR was passed over for the award, sparking debate about the gap between metrics and accolades. Front offices are increasingly using UZR as a reference metric in free agent acquisitions and contract negotiations, expanding defensive metrics from a fan's analytical tool to a business decision-making resource.
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Tracking Technology and the Future of Defensive Metrics
The accuracy of defensive metrics is directly tied to advances in data collection technology. MLB introduced Statcast across all stadiums in 2015, enabling millisecond-level measurement of batted ball speed, angle, and fielder movement speed. This gave rise to OAA (Outs Above Average), a new metric enabling more precise defensive evaluation than traditional UZR or DRS. In NPB, TrackMan was installed at all stadiums from 2020, bringing pitch data acquisition to MLB-equivalent levels, but full-stadium deployment of field tracking systems for fielder movement has not yet been achieved. Some teams have independently adopted Hawk-Eye or KinaTrax to analyze their players' defensive movements, but building a unified data infrastructure across the league remains a future challenge. Once standardized tracking data is established, NPB-specific OAA calculations, defensive shift effectiveness analysis, and real-time feedback for defensive coaching will become possible. Defensive metrics stand at a turning point, evolving from retrospective evaluation tools to real-time tactical instruments.