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A Four Factors Review of the 2019-20 Illinois Basketball Season

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What do the analytics say about the Illini?

Patrick Gorski-USA TODAY Sports

As basketball fans, we see statistics all the time. “Green Bay scored the fourth most points per game in the country last season,” you might hear. Or, “Marshall was one of the top 25 teams in the nation in defensive rebounds per game.” Surely then, Green Bay has an elite offense, and Marshall is a great defensive rebounding team, right? Not so fast. Traditional, per game statistics help to paint a picture of a team’s performance but they have one fatal flaw: they lack context. Green Bay’s offense did score lots of points last year, but this was not because its offense was great; it scored lots of points because it played a Horizon league schedule and played at an exceedingly fast pace. In reality, its offense ranked merely 120th out of 353 Division I teams (per Bart Torvik). Likewise, Marshall did grab lots of defensive rebounds, but this was not because it was a great rebounding team; it grabbed lots of rebounds because it forced lots of missed shots. When you force a lot of misses, there are more defensive rebounds available, so you will end up with lots of defensive rebounds. Marshall was actually an abysmal defensive rebounding team, ranking 328th in defensive rebounding last year, after accounting for its opponents’ missed shots.

If per game statistics can be so misleading then, what are we to do? What should we look at instead? Meet the Four Factors. In short, the Four Factors are the four different ways that a possession can end: a made field goal, a turnover, a defensive rebound, or a made free throw. The Four Factors ask these four questions:

  • How many points does a team score (or allow) per field goal attempt?
  • What percentage of a team’s (or its opponent’s) possessions end in a turnover?
  • What percentage of available offensive (or defensive) rebounds does a team secure?
  • How many free throws does a team (or its opponent) take compared to the number of field goals it attempts?

The answers to these questions are given by effective field goal percentage, offensive rebounding percentage, turnover percentage, and free throw rate, respectively. For those interested, here’s the nitty gritty math:

  • eFG% = (FGM + 0.5*3PM)/FGA
  • TO% = TO/(FGA + 0.44*FTA + TO)
  • ORB% = ORB/(ORB + Opp DRB) and DRB% = DRB/(DRB + Opp ORB)
  • FTR = FTA/FGA

The key here is that using per possession stats instead of per game stats controls for pace. Virginia’s offense will not look bad just because its pace is slow, and Nebraska’s offense will not look good just because it pushes the ball up and down the floor.

Now that we have a grasp of the Four Factors, what story do they tell about the Illini’s season?

Offense

The Illini made a more dramatic improvement on the defensive end (more on that later), but the offense improved as well, ranking 38th in schedule-adjusted offensive efficiency (Bart Torvik) this year after ranking 61st in 2019. Let’s break down the numbers.

Offensive Four Factors

Year eFG% (Rank) TO% (Rank) ORB% (Rank) FTR (Rank)
Year eFG% (Rank) TO% (Rank) ORB% (Rank) FTR (Rank)
2019 49.6% (223) 18.4% (168) 29.1% (148) 29.5% (273)
2020 48.3% (234) 17.9% (102) 35.6% (9) 32.2% (192)

Effective Field Goal Percentage

Surprisingly, Illinois’ effective field goal percentage actually got worse from 2019 to 2020, decreasing from 49.6% to 48.3%. Part of this drop can be explained by the three-point line moving back, as eFG% dipped slightly across college basketball as a whole. But even so, the Illini’s eFG% still ranked slightly lower in 2020 than in 2019.

Looking deeper, the dip in eFG% can be attributed to poor three-point shooting. The Illini actually made a higher percentage of their two-point attempts, jumping from 48.2% in 2019 to 49.6% in 2020 (250th and 166th in the country, respectively), but the three-point percentage fell off a cliff, dropping from 34.5% to 30.3% (164th and 313th).

There appear to be three main causes for the increase in two-point field goal percentage (2P%). The first is the improvement of Ayo Dosunmu. Ayo led the team in two point attempts last season, and his 2P% improved from 48.7% to 53.7%. When you take as many shots as Ayo does, a 5% increase is huge for the team. The next is the addition of Kofi Cockburn. Obviously, when you add a stud center who shoots over 50% from the floor, you will make more shots from inside the arc. The last is the improvement in offensive rebounding. We’ll look at this more later, but the offensive rebounding improvement is largely caused by Kofi. When you get a great big man, you get more offensive rebounds, and when you get more offensive rebounds, you get more high percentage looks.

I’m not really sure why the three-point shooting was so much worse. Aaron Jordan graduated, and he was the best shooter on the 2019 team, but most of his shooting was replaced by the emergence of Alan Griffin. I can’t imagine that the shooters were getting worse looks than in 2019; having Kofi Cockburn in the paint should make shooters more open on the perimeter, if anything. It seems that most of the team just got worse from three. Here are the three-point percentages of a few key players in 2019 and 2020:

  • Trent Frazier: 40.6% in 2019, 29.9% in 2020
  • Ayo Dosunmu: 35.2% in 2019, 29.0% in 2020
  • Da’Monte Williams: 31.7% in 2019, 26.8% in 2020

Somehow, Giorgi Bezhanishvili was the only player outside of Alan Griffin to shoot greater than 30% from three, making 11 of his 36 attempts on the season to shoot 30.6%.

I don’t know why the team got so much worse at outside shooting in 2020, but the team’s shooting woes more than canceled out the improvements made from inside the arc, leading to a worse eFG% overall.

Turnover Percentage

The Illini’s improvement in turnover percentage is one of the more encouraging takeaways from the 2020 season.

At the beginning of the season, the team had major turnover issues. In November and December, they had a turnover percentage of 21.6% in their 11 non-conference games. For reference, a TO% of 21.6% would have finished 321st out of 353 teams last season. The team could not stop giving the ball away. And this was against inferior competition as well.

When the calendar flipped to 2020, these issues vanished. Over the 20-game conference schedule, the Illini turned the ball over on just 15.8% of their possessions, a massive 5.8% improvement from the non-conference schedule. To be fair, most Big Ten teams play somewhat conservative defenses, preferring to stay between the ball and the hoop rather than getting in passing lanes to force turnovers. So it may be expected that the turnover percentage would decrease in conference season, even though there is superior competition. Even so, the team’s improvement, both from the 2019 season to 2020, and from the non-conference to the conference schedule, is impressive.

Offensive Rebounding Percentage

By far the biggest offensive improvement in 2020 was the offensive rebounding. After being merely average on the offensive glass in 2019, the Illini dominated the boards in 2020, ranking 9th in the country and 4th among power conference teams with an ORB% of 35.6%.

It’s not hard to see why the team got so much better at offensive rebounding; they added Kofi Cockburn. With a mark of 14.1%, Kofi finished the season with the 25th highest ORB% in the country and 11th among players in power conferences. After not having a true center in 2019, the addition of Kofi Cockburn was (quite literally) huge.

I think Alan Griffin deserves a shoutout for his offensive rebounding too. He always seemed to be making key rebounds to extend possessions, and the numbers support this. Among players 6’5” and shorter, he was 5th in the country in ORB% and 2nd among power conference players, behind only Mark Vital of Baylor. That is really, really good.

If I had to point to one thing to explain the offensive improvement from 2019 to 2020, it would be the offensive rebounding.

Free Throw Rate

Perhaps the most subtle offensive improvement the Illini made in 2020 was that they got to the line more often. The free throw rate improved from 29.5% to 32.2% (273rd and 192nd in the country, respectively).

I think just about all of this improvement can be attributed to Kofi Cockburn. Most of the returnees from the 2019 team got to the line about as often in 2020 as they did in 2019. Change nothing but add Kofi, and you will shoot more free throws. Cockburn got to the line far more often than any of the other regular players, with his 58.0% FTR nearly 20 points higher than anybody else in the rotation. Kofi ended up shooting more free throws this season than any Illini has attempted in a season since Brad Underwood arrived.

Overall Offensive Efficiency

There are two different ways to get an efficient offense: you can either make a high percentage of your shots (shot efficiency), or you can take a lot of shots (shot volume). The Illini excelled at the latter. With a mediocre eFG%, the Illini were not very efficient when shooting the ball. In fact, in conference play, Illinois ranked 13th in the Big Ten in eFG%, just 0.1% above Purdue in last. But what the Illini lacked in shot efficiency, they made up for with shot volume. The two “shot volume” statistics are TO% and ORB%. When you turn the ball over less, a higher percentage of your possessions end in a shot attempt, and when you get more offensive rebounds, you give yourself the opportunity to take multiple shots on the same possession. The Illini were very good at preventing turnovers, and they were elite on the offensive glass. Despite poor shot efficiency, the Illini’s shot volume carried them to a great offense in 2020.

Defense

The offense was marginally better in 2020 than in 2019, but the year over year improvement for the defense was night and day. After six years as a head coach, Brad Underwood ditched his signature “on the line, up the line” pressure defense this season, and it worked. After ranking 97th in schedule-adjusted defensive efficiency (Bart Torvik) in 2019, the defense shot up to 36th in 2020. Much has been made about Underwood’s schematic changes on defense, but why did they work? What went wrong in 2019, and how were those problems fixed? Let’s see what the numbers tell us.

Defensive Four Factors

Year eFG% (Rank) TO% (Rank) DRB% (Rank) FTR (Rank)
Year eFG% (Rank) TO% (Rank) DRB% (Rank) FTR (Rank)
2019 52.2% (246) 21.8% (24) 68.1% (306) 42.4% (338)
2020 47.0% (67) 16.6% (304) 74.3% (78) 23.9% (19)

Effective Field Goal Percentage

Opponents made a lot of shots against the Illini in 2019. Illinois’ effective field goal percentage against was 246th in the country and last in the Big Ten by a wide margin. It was easy to see why. The defense was based around gambling for turnovers, and in some ways, it worked; the team did force a lot of turnovers. But if they didn’t come away with the ball, too often the opponent had a wide open layup. Time and time again, the defense got beat by backdoor cuts, and when opposing ball handlers blew by defenders, the help defense was too late. When, on occasion, the opponent actually missed a shot, the defense was out of position to secure the rebound, and opponents got easy putbacks far too frequently. The young, inexperienced roster didn’t do the defense any favors either.

This all changed in 2020. Rather than focus on forcing turnovers, Brad Underwood decided to dial back the pressure and instead focus on forcing missed shots. It worked like a charm. Opponent’s eFG% dropped by more than five percentage points, one of the largest improvements in the country. Defenders had a much easier time staying in front, and when they did get beat, the help defense was there on time. Of course, having better defenders helped too. Trent Frazier came into his own on defense in 2020, and Da’Monte Williams, Andres Feliz, and Ayo Dosunmu were rock solid defending the perimeter. And, with all due respect to Giorgi Bezhanishvili, Kofi Cockburn was a much more imposing rim protector than Giorgi ever will be. The scheme kept rebounders in position as well, preventing opponents from getting many easy looks at the rim. Combine a scheme designed to force misses with Cockburn and improved perimeter defenders, and you get a dramatic improvement in defensive eFG%.

Turnover Percentage

The one part of the defense that got worse from 2019 to 2020 was the dropoff in opponents’ turnover percentage. After ranking 24th in 2019, Illinois fell all the way to 304th in defensive TO% in 2020.

While you want to force turnovers as a defense, this dropoff was intentional. It is really hard (but not impossible) to be great at both forcing misses and forcing turnovers. More often than not, a team that forces lots of missed shots will not create many turnovers and vice versa. Last season, Brad Underwood and the coaching staff decided that the team would be best served by playing more conservatively and focusing on forcing missed shots. After jumping from 97th to 36th in the country in defensive efficiency, it’s hard to argue with the results.

Defensive Rebounding Percentage

Defensive rebounding was quietly a weakness for the Illini in 2019, but it turned into a major strength for the team in 2020. After being among the worst in DRB% in the country in 2019, it jumped up to 78th this year.

I think Illinois’ struggles on the defensive glass in 2019 often got lost, overshadowed by other defensive problems. It was easy to see when the defense consistently allowed wide open looks. It was easy to see when opponents lived at the free throw line. The rebounding issues were a bit more subtle. I don’t think the rebounding issues were for a lack of skill. Sure, the team didn’t have a true center, but Feliz, Dosunmu, Williams, Bezhanishvili and Griffin were all perfectly capable rebounders. I think the root of the issue was with the scheme.

In Brad Underwood’s pressure defense, off-ball defenders are instructed to play “on the line,” meaning they should stay between the ball and their man. Playing on the line gets defenders in passing lanes and forces more turnovers, but it causes problems on the defensive glass. Defenders are farther away from the hoop, and they are not in a good position to box out.

When Underwood went to a gap-style defense in 2020, he not only forced more missed shots, he fixed the rebounding issues too. When defenders play between their man and the hoop (instead of between their man and the ball), it obviously makes it easier to prevent open looks, but it also makes it easier to get defensive rebounds. Defenders don’t have to scramble to get in front of their man to box out; they are already there.

Because of Brad Underwood’s schematic changes, the Illini went from a terrible rebounding team to a great one. Oh, and adding Kofi Cockburn helped too.

Free Throw Rate

The biggest improvement on offense or defense in 2020 was in defensive free throw rate. After ranking in the bottom 20 in the country in 2019, the Illini jumped all the way up to the top 20 in defensive FTR in 2020.

Fouling too much was always one of the consequences of Underwood’s turnover-centric defensive scheme; his teams ranked 327th or worse in defensive FTR in five of his first six seasons as a head coach. His teams were great at forcing turnovers as we saw, but gambling for steals often led to fouls. All of the fouls hurt the team directly by sending opponents to the line, but it also kept the best players off the floor with foul trouble, hurting the team on offense as well as defense.

After Underwood changed his defensive philosophy in 2020, all of the fouling issues were fixed. I don’t even think much, if any of the improvement can be attributed to the personnel; the only major addition was Kofi Cockburn, and he was one of the most foul-prone players on the team. I think the credit here lies solely with Underwood and the coaching staff.

I should note that while the team got better in part because it fouled less often, the causation runs the other way too. Teams that foul less often win more games for obvious reasons, but teams that win more games foul less often as well, because they don’t need to foul to come back late in games. Nevertheless, credit to the coaching staff for completely erasing the team’s excessive fouling.

Overall Defensive Efficiency

This season, Brad Underwood decided to focus on forcing missed shots instead of forcing turnovers. In effect, he traded a bad eFG% against and a great defensive TO% for a good eFG% against and a terrible defensive TO%. If we stop here, this trade is probably a wash, or maybe even a small loss. But with this trade came a good DRB% and a great defensive FTR as well. All of a sudden, the trade looks great. On defense, the Illini went from being great at one thing (forcing turnovers) and terrible at everything else in 2019 to terrible at one thing (again, forcing turnovers) but great at everything else in 2020. That seems like a good trade to me. And with the defense improving from 97th in the country to 36th, the results speak for themselves.

Conclusion

The Illini’s improvement from 2019 to 2020 was truly remarkable. A season after posting its worst record since 1975, Illinois came within one game of the Big Ten title in 2020, and no power conference team improved its record by as much as Illinois did last season. Much of the improvement can be attributed to an improved roster; returning nearly everyone in the rotation and adding a top-50 freshman is a formula for success. But let’s give credit where credit is due. We Illini fans love to question our head coaches, but Brad Underwood and his staff did a fantastic job with this team. Many coaches are stubborn and refuse to change their philosophies, even when they are not working. Not Brad Underwood. He saw that his pressure defense wasn’t working, so he ditched it. Thanks to Brad Underwood, Illinois Basketball is back.