Saturday night, somebody noticed that Saint Louis had a horrible defensive 3P%. The first effects of the post-Rick Majerus era, right? Not really. It’s because the defense has little control over its opponents’ 3P%. As an illustration, let’s look at the best and worst “three-point defenses” in previous seasons on December 4th, and examine how they fared the rest of the season.

What I’ve done is taken both the top 20 and bottom 20 teams in opponents’ 3P% as of 12/4 each of the last four seasons, and recorded their three-point defense from December 5th onward. All figures are averages for the 20 teams in each group.

          Thru 12/4             After 12/4
      Top 20   Bottom 20     Top 20   Bottom 20   
2012   45.2       23.8        34.8      32.9                   
2011   44.6       24.7        34.6      33.3
2010   45.1       24.6        35.4      34.0
2009   44.5       23.9        34.3      34.2
 Avg   44.9       24.3        34.8      33.6

So on average over the rest of the season, there will be a roughly a 1-2 percent difference between the teams currently in the top and bottom 20 of opponents’ 3-point percentage. We’re talking about a difference of one make every two or three games between the best and worst groups. Without more analysis, one can’t say precisely how much skill a team has at influencing its opponents’ three-point percentage, but there’s a fair amount of evidence it’s not much, and even end-of-season figures are the result of significantly more noise than skill.

With few exceptions, the best measure of three-point defense is a team’s ability to keep the opponents from taking 3’s. This is what the Majerus defense did, and fortunately for Billiken fans, Jim Crews is carrying on that tradition as SLU is currently allowing the nation’s second-fewest 3PA%.

When people say that advanced-stats users are a bunch of nerds, I can only think the people that don’t use them are the real dorks. Nobody with any knowledge of the game would talk about free throw defense using opponents’ FT% as if it was a real thing, yet we’ll hear plenty of references to three-point defense in that way from famous and respected people. Of course, both free-throw defense and three-point defense exist, but it’s much better measured in the amount of shots taken and not in the noisy world of the percentage of shots an opponent makes.

For reference, here’s a rough analysis of how well various team shooting stats currently identify greatness (or awfulness). The left column is the 12/4 difference between the average of the top 20 and bottom 20 in each category, and the right column is the difference between the same groups of teams ove the rest of the season. Essentially, the larger the difference going forward, the less noise the stat contains. Only data from last season is used in this comparison.

            Pre 12/4      Post 12/4
              Diff           Diff   
Opp FT%       19.7            1.7
Opp 3P%       21.4            1.9
Own 3P%       21.7            3.6
Opp 2P%       20.9            4.8
Own 2P%       19.6            5.5
Own FT%       22.1            8.5
Opp 3PA%      20.7            9.7
Own 3PA%      24.1           14.5 

When someone discusses three-point defense in terms of three-point percentage, they might as well make the leap to discuss free-throw defense in similar terms. Teams have much more control over how many three’s their opponents shoot than how many they make.