Defensive Win Shares Are Completely Broken

As the flagship one metric stat of the popular website basketball-reference, win shares are heavily cited in the  basketball world. Unfortunately, the stat is far from perfect, and the defensive side of the stat is particularly egregious. Defense is notoriously tricky to quantify, and win shares attempts this using the usual bevy of defensive box score stats like blocks and steals, but they also include the team's defensive rating. This means that Zach Randolph gets the same credit for defense Tony Allen and Marc Gasol do, ignoring the few defensive box score stats. The influence of team defense is huge on this stat. What's even more problematic is that it's used in the time before steals and blocks, and when defensive rebounds weren't tracked separately from total rebounds.

The best example of how the metric fails is with Ryan Anderson. Traded from the Magic, who with Howard were a perennial defensive team, to the Hornets, still reeling from tanking and poor decisions, what changed was his scenery, not his defensive skill. Obviously, motivation is important with defense, as is coaching, but his defensive rating tracks closely to this team's rating. 2012 was his breakout season, yet his defensive rating plummeted from 19th in the league, or nearly a 95th percentile, to almost exactly average. The table below has the full details where percentile is based on players that season with at least 500 minutes.

Ryan Anderson's defensive rating (win shares)

Season	Defensive rating	Defensive rating percentile	Team rating	Team ranking
2011	101	94.4	101.8	3
2012	105	51.1	104.1	12
2013	112	7.0	110.3	28

When people use defensive win shares, or win shares in general, they may not fully understand how it's calculated. This is the clearest way to illustrate that.

Need another example? Omer Asik was traded from the defensively dominant Chicago Bulls to the Houston Rockets, where he has to adjust for the mistakes from James Harden and a roster of rookies. Few people watching Asik this year would say he has dropped off significantly in defensive intensity or skill. However, the previous year his rating was 2nd and before that, his rookie season, he was 3rd. This season? 68th. The Rockets are a league average defensive team because of him; when he's off the court they're terrifyingly bad, yet win shares only sees a player who's a great defensive rebounder with some blocked shots on a mediocre defensive team.

Omer Asik's defensive rating (win shares)

Season	Defensive rating	Defensive rating percentile	Team rating	Team ranking
2011	97	99.1	100.3	2
2012	92	99.4	98.3	2
2013	103	80.0	106.1	16

You can see this with players traded midseason too: Tayshaun Prince went from a defensive rating of 111 to 103, effectively going from roughly one of the worst defenders in the league, roughly 10th percentile, to significantly above average. (And no, it's not effort: win shares only sees box score stats, as his defensive rebounding leveled off but his steals and blocks went up, but the major culprit to the change was Memphis' suffocating defense compared to Detroit's mess.)

Need a quick way to discredit the stat to someone? In 2013, DeJuan Blair was 7th in the league in defensive rating.