Measuring defensive contribution by how much expected threat (xT) a defender prevents rather than by counting tackles, interceptions, or clearances. The key insight: "If you have to make a tackle, you've already made a mistake" (Maldini). Good defending means the opponent never reaches high-threat zones — the xT potential of the attacking team stays low. By integrating xT over the zones the attacking team controls (using tracking data) or by measuring the xT at the point of defensive actions (using event data), you can quantify how much threat each defender prevented.
With tracking data: At each frame, compute the xT potential of the attacking team by integrating xT over the zones they control (based on player positions and Voronoi tessellation). When a defender's positioning forces the attack into lower-threat zones, the xT potential decreases — that decrease is attributed to the defender. Compare: (1) xT potential when defender is well-positioned (organized block, compact shape), (2) xT potential when defender is sucked out of position (disorganized, gaps open). The difference is the defensive contribution.
With event data (simpler): For each defensive action (tackle, interception, pressure), compute the xT of the zone where the opponent was when the action occurred. High-xT defensive actions (stealing the ball in dangerous zones) are more valuable than low-xT defensive actions (winning the ball at the halfway line). But also measure: was the opponent IN a high-xT zone because the defender allowed them to get there? A tackle in the box might indicate a failure upstream.
The Maldini principle — "if you have to make a tackle, you've already made a mistake" — is quantifiable via xT denied. Defenders who make the most tackles and interceptions are typically doing so in high-xT zones near the box, meaning they allowed the opponent to penetrate that far. The best defenders never need to tackle because opponents never reach dangerous zones.