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.
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.
Unique Difficulties in Quantifying Outfield Defense
Compared to infield defense, quantitative evaluation of outfield defense presents unique complexity. Outfielders cover vast territory with longer hang times on batted balls, making first-step decisions and route selection critical to recording outs. The outfield component of UZR calculates play difficulty from the relationship between the landing point and the fielder's starting position, but correcting for environmental factors such as wind direction, temperature, and park dimensions remains challenging. The sea breeze at Koshien Stadium and the ocean wind at Mazda Stadium significantly alter batted ball distance, meaning identical fly balls carry different difficulty ratings across parks. Fence distance and height also determine whether diving catches are feasible, requiring precise park-factor adjustments for cross-stadium comparisons. Post-catch throwing evaluation is similarly complex, as assist rates depend partly on runners' judgment rather than solely on arm strength.
The Relationship Between Defensive Metrics and Salary Arbitration
The influence of defensive metrics on NPB salary negotiations became visible around 2018. Traditional salary evaluations centered on batting statistics (batting average, home runs, RBIs) and pitching statistics (ERA, wins), with defensive contributions assessed largely by subjective impression. However, as publicly available UZR data proliferated, player agents began citing UZR in contract discussions. For example, a shortstop with a low batting average but strong positive UZR might present defensive contributions in monetary terms during negotiations. Multiple reports indicate that front offices now include UZR as a reference metric in internal contract renewal documents. However, UZR can fluctuate significantly year to year, with the possibility of sharp declines the following season, making multi-year averages the preferred evaluation basis. No NPB team has officially incorporated defensive metrics into formal salary arbitration criteria, and they remain supplementary decision-making tools.
The Debate Over Defensive Shift Effectiveness
MLB banned extreme defensive shifts starting in 2023, but NPB has not introduced explicit shift regulations. Adoption of defensive shifts in NPB began around 2016 among certain teams, with an increasing number designing optimal positioning for each batter based on Data Stadium Inc.'s spray charts. Attempts have been made to apply UZR methodology when evaluating shift effectiveness, comparing out-conversion rates on batted balls to the same zones under shifted versus traditional alignments to estimate run-prevention value. However, because detailed shift-deployment data is not publicly available in NPB, analysis largely depends on proprietary team information. The players' union has raised concerns that shifts reduce hit totals and thereby depress salary evaluations for batters, and discussions continue regarding how to adjust batting metrics to account for shift-influenced outcomes.