I Calculated NPB Park Factors for 10 Years — Stadium Renovations Revealed

What I Built Using 8,619 NPB (Japanese professional baseball) game scores from 2016–2025, I calculated Park Factors (PF) for all 12 stadiums and visualized the year-by-year trends. The key focus: how do stadium renovations change the numbers? → GitHub : https://github.com/yasumorishima/npb-prediction What Is a Park Factor? Park Factor measures how much a stadium affects run scoring compared to a neutral park. Formula (Baseball Reference standard): PF = ((Home RS + Home RA) / Home G) / ((Away RS + Away RA) / Away G) PF > 1.00 : Hitter-friendly (more runs scored) PF = 1.00 : Neutral PF < 1.00 : Pitcher-friendly (fewer runs scored) Why Not Just Use Home Runs Scored? # ❌ This mixes in the team's offensive strength pf_bad = home_score / average_score # ✅ Comparing the SAME team's home vs away performance isolates the park effect pf = ( home_RS + home_RA ) / home_G / (( away_RS + away_RA ) / away_G ) By comparing how the same team performs at home versus away, we isolate the stadium's contri