Should We Ignore The Components Of Defense That Teams Can't Control?

[RealGM Articles]

Given the small sample of games in a college basketball season, even if every player returns, a team’s defense is surprisingly unpredictable. As an anecdotal illustration of that point, in last week’s column I noted that North Dakota St. brought back essentially its entire roster in 2014, and yet NDSU’s defense was much worse than in 2013. But brilliant South Carolina Gamecock writer @chickenhoops emailed me and pointed out that North Dakota St. wasn’t necessarily a worse defensive team last year, they were just unlucky. The Bison’s collapse on defense was almost entirely driven by factors outside their control. Notably, North Dakota St.’s opponents hit an incredibly high percentage of three pointers, and made an extremely high percentage of free throws on the season. I had noted that North Dakota St.’s defense fell from 59^th to 131^st. But ChickenHoops noted that if opponents had made the same percentage of threes or the same percentage of free throws as the average D1 team, North Dakota St.’s defense would have ranked 70^th in 2013 and 74^th in 2014.

Most people would agree that teams have little control over their opponent’s free throw percentage. And Ken Pomeroy has argued that an opponent’s three point percentage is something teams also have little control over. (Ken argues that teams have control over whether opponents take threes, but have substantially less control over whether those shots fall.)

And as ChickenHoops proposes, we can easily recalculate each team’s points allowed per 100 possessions, assuming their opponents hit the D1 average percentage of free throws and threes. This new measure of defense will be the points allowed per 100 possessions, assuming more typical luck. In the next table I show how each team’s defense would have looked last season using this new measure.

In the major conferences, a team like Vanderbilt stands out as fortunate last year. While the average team made roughly 70% of its free throws last year, Vanderbilt’s opponents made only 65% of their free throws. And while the average team made 35% of its threes last year, Vanderbilt opponents made only 30% of their threes.

On the flip side, a team like Notre Dame was probably very unfortunate to have such a bad defensive season last year. Opponents made 75% of their free throws and 39% of their threes against Notre Dame last year, far above the national averages.

Team	Conf	Def	New Def	Change
Vanderbilt	SEC	99.0	101.3	2.3
Clemson	ACC	95.0	97.2	2.2
Memphis	AAC	98.1	100.0	1.9
Texas A&M	SEC	96.3	98.1	1.8
Louisville	AAC	90.0	91.8	1.8
Arizona St.	P12	97.6	99.4	1.8
Northwestern	B10	94.2	95.9	1.7
Kansas St.	B12	94.8	96.4	1.6
North Carolina	ACC	95.4	96.9	1.5
Virginia	ACC	90.1	91.6	1.5
Virginia Tech	ACC	100.5	101.9	1.4
Nebraska	B10	96.1	97.4	1.3
Connecticut	AAC	91.8	93.1	1.3
Cincinnati	AAC	91.3	92.5	1.2
Utah	P12	96.5	97.6	1.1
USC	P12	102.2	103.2	1.0
Ohio St.	B10	89.6	90.6	1.0
Butler	BE	99.6	100.6	1.0
Wake Forest	ACC	101.9	102.8	0.9
South Carolina	SEC	102.3	103.2	0.9
Syracuse	ACC	93.6	94.4	0.8
Duke	ACC	102.3	103.1	0.8
SMU	AAC	94.7	95.5	0.8
UCF	AAC	106.1	106.9	0.8
Missouri	SEC	104.4	105.2	0.8
Oklahoma St.	B12	96.6	97.4	0.8
Kentucky	SEC	96.9	97.7	0.8
Maryland	ACC	95.5	96.2	0.7
Indiana	B10	97.5	98.2	0.7
Washington	P12	104.5	105.2	0.7
Xavier	BE	100.6	101.3	0.7
West Virginia	B12	104.2	104.8	0.6
UCLA	P12	97.3	97.9	0.6
Miami FL	ACC	100.0	100.6	0.6
Providence	BE	102.2	102.8	0.6
Florida St.	ACC	98.8	99.4	0.6
Iowa	B10	102.7	103.3	0.6
Florida	SEC	89.3	89.8	0.5
Baylor	B12	100.0	100.4	0.4
Georgia	SEC	99.1	99.5	0.4
Kansas	B12	96.3	96.7	0.4
Georgetown	BE	102.1	102.5	0.4
Pittsburgh	ACC	96.2	96.5	0.3
Mississippi	SEC	102.7	102.9	0.2
Marquette	BE	100.2	100.4	0.2
Creighton	BE	104.1	104.3	0.2
Temple	AAC	109.1	109.3	0.2
Mississippi St.	SEC	103.7	103.9	0.2
Georgia Tech	ACC	99.8	99.9	0.1
Oregon	P12	100.6	100.7	0.1
Purdue	B10	101.2	101.3	0.1
NC State	ACC	102.9	103.0	0.1
Arizona	P12	88.5	88.6	0.1
Arkansas	SEC	98.1	98.1	0.0
Illinois	B10	93.3	93.3	0.0
Texas	B12	98.4	98.4	0.0
Michigan	B10	102.1	102.1	0.0
TCU	B12	103.1	103.0	-0.1
Houston	AAC	108.0	107.9	-0.1
Tennessee	SEC	94.8	94.7	-0.1
St. John's	BE	96.8	96.6	-0.2
Washington St.	P12	103.5	103.2	-0.3
LSU	SEC	99.4	99.1	-0.3
Oklahoma	B12	100.6	100.2	-0.4
Colorado	P12	96.9	96.5	-0.4
Wisconsin	B10	97.6	97.2	-0.4
Villanova	BE	94.5	94.1	-0.4
Oregon St.	P12	107.1	106.6	-0.5
Seton Hall	BE	101.2	100.7	-0.5
Alabama	SEC	100.0	99.5	-0.5
Michigan St.	B10	96.2	95.7	-0.5
Stanford	P12	97.0	96.4	-0.6
Penn St.	B10	100.8	100.2	-0.6
Iowa St.	B12	99.9	99.2	-0.7
DePaul	BE	107.3	106.5	-0.8
Texas Tech	B12	102.2	101.4	-0.8
Auburn	SEC	106.5	105.7	-0.8
Minnesota	B10	100.4	99.6	-0.8
South Florida	AAC	104.1	103.2	-0.9
California	P12	100.6	99.6	-1.0
Rutgers	AAC	106.3	104.9	-1.4
Boston College	ACC	111.4	109.9	-1.5
Notre Dame	ACC	106.4	104.1	-2.3

Though not listed in the above table, Wichita St. was also extremely fortunate last year. The Shockers opponents made only 65% of their free throws and 31% of their threes last year. Wichita St.’s opponents rarely had scorching shooting nights, and when they did, (Evansville making 5 of 11 threes and 15 of 16 free throws), the game was enough of a mismatch that it didn’t matter. The Kentucky game was arguably the first time on the season that one of Wichita St.’s quality opponents had an unusually good shooting night.

This is worth revisiting in the middle of next season, to see if the margin-of-victory rankings are misleading us about the top teams. But it can make a meaningful difference. With this alternative metric, Notre Dame was not the 99th best team in the nation last year, but they were the 82nd best team.

Another way to think about this is to think about performance between seasons. A team’s adjusted defense is correlated year-to-year. But if I use this new measure of defense, the year-to-year correlation is meaningfully higher. The improvement in the unexplained variation is a bit difficult to put into words, but perhaps a comparison will help. One of the factors that matters in my model is the height of each team’s center. Using this new measure of defense improves the performance of my prediction model four times as much as adding the height of each team’s center to the model.

Since my prediction model is a bit complicated, for now let’s think about the simplest possible prediction model which accounts for returning minutes (but which does not account for individual player stats, player heights, recruiting rankings, or coaching). I’m discussing results based on the last 10 seasons of data. In this simple prediction model, if a team brings back the average number of minutes, having a defense that is 1 point better predicts that the defense will be 0.67 points better the following season. But when we use this new measure of defense, a defense that is 1 point better predicts that the defense will be 0.72 points better the following season. For a team on the unfortunate end of the scale, like Notre Dame, using the new measure of defense essentially moves them up about 12 spots in my projections for next year.

It is not fair to say that teams have no control over their opponent’s free throw percentage. FT defense depends on which players a team fouls. And Pomeroy has admitted that three point percentage includes some information. But college basketball analysts should be thinking of this a bit like Batting Average on Balls in Play in baseball. If a team is way too high or way too low in free throw defense or three point percentage defense, that probably is a bit about luck. Both within seasons and between seasons, we shouldn’t necessarily expect that component of defensive performance to persist.