Important Defensive Stats for the Spurs

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All stats were from before the Spurs’ Wednesday night game against the Phoenix Suns.

Dean Oliver, author of Basketball on Paper, a well-known book about advanced stats in the NBA, researched which metrics were most important to winning and published them in both his book and in an article online in the early 2000’s. Oliver called these metrics the Four Factors. The Four Factors are effective field goal percentage, turnover percentage, offensive rebounding percentage, and free throw rate. While these four factors look at a team’s offense, looking at opponents’ four factors can give insight into a team’s defense.

Which of the four factors are most important on defense? To find out, I found the correlation between each of the defensive four factors and winning and defensive rating in the 2017-2018 NBA season. Opponents’ effective field goal percentage had a moderately strong negative correlation (-0.54) with winning, meaning that the lower a team’s opponents’ average effective field goal percentage was, the higher that team’s total win count was. The other three factors had a fairly weak correlation to winning, with opponent’s free throw rate having a negative correlation to winning of -0.23, opponents’ turnover percentage having a correlation to winning of +0.21, and opponent’s offensive rebounding having a correlation to winning of -0.04.

Defensive rating was affected differently by most of these factors. Opponents’ effective field goal percentage had a very strong correlation (+0.82) with defensive rating. Opponents’ offensive rebounding had a +0.30 correlation with defensive rating, opponents’ turnover percentage had very similar correlation with defensive rating as with winning at -0.21, and opponent’s free throw rate had a weaker correlation with defensive rating at +0.10.

Unfortunately for the San Antonio Spurs, the factor that had the highest correlation with winning and defensive rating, opponents’ effective field goal percentage, is a defensive metric they rate poorly in. Through October 30th, the Spurs ranked 28th in opponents’ effective field goal percentage at 56.1%. Last season, they produced a more favorable number in this metric, ranking 8th while holding opponents to an effective field goal percentage of 50.7%.

The Spurs’ are already strong in limiting opponents’ free throws. They rank first in the league in opponent free throw rate at 0.211 free throw attempts from opponents for each of their field goal attempts. The Spurs are an average team so far at forcing turnovers, ranking 17th and causing turnovers on 14.4% of opponents’ possessions. So far they rank 15th in opponents’ offensive rebounding percentage, allowing offensive rebounds on 26.7% of opponents’ own misses. The rebounding numbers in particular should improve as Derrick White and Lonnie Walker IV return, and the Spurs are able to play bigger than most teams due to roster construction.

This means that the Spurs can be average or better at three of the four defensive factors. Unfortunately, the one factor that they may have a difficult time improving is the most important factor, opponents’ effective field goal percentage. Limiting threes is one way the Spurs can lower this number, however. The Spurs allow the 10th most non-corner three attempts, according to CleaningTheGlass.com. Last season, they ranked 4th best in this metric.

While the Spurs may have defensive struggles throughout the season, they should be stronger on offense. The Spurs currently rank 9th in offensive rating at 111.1 points per 100 possessions, better than last season’s ranking by 4 points per 100 possessions. The Spurs can find relief in this as last season offensive rating had a very strong correlation with winning at +0.84.

All stats are from NBA.com/stats unless stated otherwise.

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