NBA home court losing its value

— -- Stumped by the disappearing home-court advantage in the NBA?

So were we. We proposed some theories in this space here, but we decided to get our hands dirty and dig into the data.

Here's what we found. Let's tackle this FAQ style.

OK, so how much has home court really mattered?

Simply put: a lot. If a picture is worth a thousand words, then the following chart speaks volumes:

As you can see, the home-court edge has more or less been in straight decline over the past few decades, but it typically hovers just above 60 percent. However, this recent two-year drop has been enormous. So far this season, teams are winning just 53.7 percent of the time. That's down from 58 percent last season and 61.2 percent the season before that. Since 1975, the advantage has been an average of 61.2 percent. The peak? In 1988-89 it stood at 67.9 percent.

You might have heard that home-court advantage is worth at least 3.5 points per 100 possessions in the NBA. And that was true for a long time. But not this season (or last). This season, home-court advantage has stood at plus-2.2. Last season, it was plus-2.6. The season before that: plus-3.2. If we adjust for pace, the two-year decline is 36.7 percent, or more than one-third. In just two seasons. We've never seen such a dramatic two-year decline.

Is the decline in home-court advantage statistically significant?

It is. The large observed drop in home-court advantage since the start of the 2013-14 season is something that we would only expect to observe by random chance less than 1 in 1,000 times.

In other words, it's statistically significant. (For those interested in some gory details: We can calculate the odds of seeing a sample mean of any given value -- in this case, the mean home-court advantage over the sample of 1,897 games since the start of last season -- because the distribution of all possible sampled means is approximately normal with a standard deviation inversely proportional to the square root of the number of games in the sample. In a database of thousands of games dating back to 1996, we observed a per-game standard deviation in home-court advantage of 14.1, which yields a standard deviation of sample means equal to 14.1/(1897^0.5), which works out to a standard deviation of 0.324 points/100 possessions. So, the recent drop in home-court edge -- more than 1.0 point per 100 possessions -- is a shift of more than three standard deviations in magnitude, which is an event that occurs by random chance less than 1 in 1,000 times.)

Using a similar methodology, we also found statistically significant declines in the home-court edge in effective field goal percentage, offensive rebounding and free throw rate since the start of the 2013-14 season.

How many teams are worse at home?

One-third of the league. No joke.

No HCA full season

Right now, 10 teams have fared no better at home than on the road. That's a mind-boggling number. To wit, only one team -- the Washington Wizards -- finished last season with a worse or equal record at home compared to the road. Just one.

And get this: In the previous eight seasons, there were only 10 teams combined that saw this reversal effect. Again, we have 10 this season. Brooklyn, Chicago, Dallas, Detroit, Houston, Miami, Milwaukee, Minnesota, Orlando and Philadelphia have not posted a better home record than on the road. If you pull up the standings in 1997-98, there were no such teams. Same in 1998-99. Same in 1999-00. Same in 2000-01. Four straight seasons, not one team. And now we have 10.

See the chart to the right that outlines the number of teams that had no HCA for a full season.

You might be asking yourself, "Well, why don't we compare apples to oranges and just look at this point in time each season? Same effect?" Great question. We looked that up, too. Through each team's first 50 games (about where we are now) since 1997, we have never seen five teams fail to do better at home than on the road at this juncture. Again, there are 10 teams now.

The chart below shows the number of teams with no HCA at the 50-game mark.

No HCA 50-game mark

So even if we just look at this point in the season historically, this season is screaming anomaly.

Is there anything unique about these 10 teams?

Hard to find anything. Geographically, maybe? Looking at the list, most of the teams are in the middle of the country. No West Coast teams. But that's a stretch. There are good teams: Houston, Dallas and Chicago. And there are bad ones: Minnesota, Orlando and Philadelphia. They come in all shapes and sizes.

Does the league's 3-point frenzy have something to do with it?

Here's where it gets interesting. You may have noticed that 3-pointers are on the rise. But what you may not know is that for the first time in NBA history, we will see more 3-point attempts than free throw attempts this month. It's all happening right now. Don't believe me? Check out the chart below.

Here we have a situation where teams are trying to win by way of the 3-pointer rather than attacking the rim and trying to get to the line. The NBA is a much more open game now than it was in the 1990s. And that style of play has ripple effects. You're not drawing as many fouls if you're chucking from the perimeter. With more outside jump shooting, the two teams are deciding the game with their play rather than subjecting themselves to the quirks of the third party: referees.

With all the perimeter shooting, it stands to reason that referees could have less influence on the game. And studies have shown from other sports like soccer and baseball that referee bias (the subconscious desire to please the crowd) isn't just a sore loser's concoction. If an increase in 3-pointers removes referees from the equation somewhat, maybe home-court advantage goes by the wayside.

Another ripple effect of 3-point shooting: more variance in close games. If an underdog (traditionally, the visiting team) is trying to win, it behooves it to try riskier strategies. Three-pointers may not splash through the net as often as a 2-point attempt or a free throw attempt, but that risk comes with the higher reward.

OK, what about winning time, the clutch?

Ah, yes, the clutch stuff. For as long as the NBA has tracked in-game trends, the home team has enjoyed a nice cushion in clutch situations. What do we mean by clutch situations? We'll go with the NBA.com definition: game within five points in the final five minutes of the contest. Here is where the home-court advantage comes up big. That is, until now.

Entering this season, the home team has won 54.8 percent of its games in clutch situations since the league began tracking this data back in 1996-97. And this makes sense. All else equal, it should be a random coin flip. But everything's not equal. The home crowd gets louder. Opposing teams might buckle under the pressure with the game on the line. Road legs might get weary. From 1996-97 to 2013-14, we didn't see a single season where home-court win percentage finished below 51.6 percent.

This season? The home team has won just 47.7 percent of the time. That's right. The home-court advantage has evaporated and flipped the other direction. Visiting teams have pulled out more close wins than home teams this season.

Whoa. And this isn't a small sample size. We're talking more than 350 games this season where we've seen clutch situations and the visiting team has won just 183 of them. What about when the game gets tighter, you ask? Let's look at "superclutch" situations where the game is within three points in the final three minutes. Yep. Same story. Visiting teams buck the trend and win more often than the home team. The home-court team has fallen below .500 for the first time on record: 48 percent in superclutch. In "superduper clutch" (terrible name, we know) where it's a one-possession game in the final minute, the home team has won just 48.5 percent. Again, consistently the home team has had the historical advantage here with the game on the line. Not this season.

What about the "tanking" teams in the clutch?

This came up in the research. You know who the worst home teams are in the clutch? The usual suspects: The Knicks (3-9; .250), Lakers (3-11; .214), 76ers (2-8; .200) and Timberwolves (2-10; .143) rank last in win percentage on their home floor in clutch situations. Collectively, they're 10-38 (.208) in clutch situations this season on their home floor. If you remove them, the home-court win percentage jumps to .521. But if you removed the worst four teams in the NBA every season, the win percentage would skyrocket, too. Nothing weird there. Just a contributing factor to the home-court reversal in tight games.

OK, well what about this time last season and the seasons before that?

You might have thought this is just a midseason blip, that it's too early to say anything definitive. But what we're seeing now is not typical at the 50-game mark. Even just looking at in-season trends, the home-court advantage we're seeing now is still abnormally small. As you can see in the chart below, the cumulative home-court advantage through a team's first 50 games has not been lower than it has the past two seasons. All the gray seasons are historical ones dating back to 1997 with the exception of 2013-14 and this season, which stand out. Before that? Nothing close to what we're seeing these days.

You can tell by the black line that the overall average doesn't take on a U-shape as the season progresses. However, it did last season. Maybe the line moves back toward the black line, but it's doubtful it surges all the way to the historical norm. We're in too deep.

Did home-court advantage bounce back last postseason?

Interestingly enough, it did not. Home-court advantage all but disappeared in the playoffs, too; the effect wasn't quarantined in the regular season. The home team went 50-39, winning 56.2 percent of playoff games in 2014. That seems like a healthy edge, but not when we zoom out and find that the home team has enjoyed a 65.1 percent cushion since 1997.

Look in the data and you'll find that home-court advantage typically strengthens in the playoffs. But last postseason, home-court advantage actually shrank. In the 2008 playoffs, the win percentage of home squads stood at 74.4 percent. Once we adjust for pace, the 2008 home team enjoyed a 9.3-point edge on average in the playoffs. Last season, that edge fell to just three points. Yes, sliced down by two-thirds. Crazy, right?

Has Vegas caught on?

Not really. Home teams have underperformed the Vegas lines all season. According to covers.com, the away team is 362-300-16 (54.7 percent) against the spread. If Vegas was 100 percent accurate, it would be a flat 50 percent, but they're still underrating the road team.

Are home teams getting called differently this season?

But there's another school of thought here: Home fans like to feel like they matter. If the home crowd no longer feels like the proverbial sixth man on the floor, will they keep showing up to the game? Why spend your time cheering and jeering if the results show that cheering and jeering aren't as rewarding as they used to be?

Well, because it's fun to see a professional basketball game live and it's still a rush to support your team. You cheer your favorite band at a concert not because it'll make them hit that note a little crisper, but because you're a fan and you want to let them know.

If home-court advantage means little, why fight like mad for seeding throughout an 82-game season?

If you think teams rest their guys leading up to the playoffs now, just wait until they don't think home-court matters. Because what's the point? It's true, the NBA is a business and teams want to get their hands on the gate revenue for that potential seventh game in a series.

But coaches might see it differently. If San Antonio knows that home court isn't worth much anymore, do the Spurs care whether they get in as the fourth seed or the eighth seed? The regular-season mentality might evolve into "just get to the playoffs" as opposed to "get the best seed possible." Is that good or bad for the product in the long run?

Ultimately, if the NBA is cracking down on unfair referee calls, that has to be seen as a good thing. But a move toward fairness might lead to some tricky unforeseen consequences.

We want you to join the conversation. What do you think explains what's going on? And would this be a good thing? We'd love to hear your thoughts. Use the hashtag #HomeCourtDisadvantage and hit us up at @ESPNInsider, @ESPNNBA or @tomhaberstroh on Twitter.

The effect gets stronger when you look at not just road teams, but good road teams. If you've picked the away favorite against the spread, you have won 56.7 percent of your bets (where 52.3 percent is the standard break-even rate of success). Monster edge.

What if bad teams have played a ton of home games? Wouldn't that explain it?

Well, it hasn't been the case. It's the opposite, actually. The Clippers have endured the most home-heavy schedule in the league, playing 27 of their 45 games at home. And they're a great team.

The two juggernauts -- Golden State and Atlanta -- have both played more home games than away. Actually, using ESPN's Basketball Power Index (BPI), there is a positive correlation between good teams and games played at home thus far. So, that doesn't explain what's going on. If anything, the home teams have a worse record this season, despite a bunch of good teams playing home-heavy schedules.

Are home teams getting called differently this season?

We're just scratching the surface of the play-by-play data right now, but some interesting trends have popped up. I asked Daren Willman, the guru behind the indispensable NBAsavant.com play-by-play database, to send over some data on referee foul calls dating back to 2010-11.

I examined shooting fouls in particular. This season, the away team is called for 3.5 percent more shooting fouls than home teams, which is down from 4.9 percent in 2013-14 and 5.4 percent in 2012-13. The gap is apparently shrinking. Shooting fouls are being called more evenly now than in recent seasons.

However, personal (non-shooting) fouls have not followed the same trend. In fact, it seems to be going in the opposite direction. In 2012-13, the home team was called for the same number of personal fouls on a per-game basis. But last season, the visiting team was called for 0.5 percent more personal fouls than the home team. This season? It's jumped to 3.8 percent. When we look at personal fouls, the home team seems to receive more calls, if slightly. In other words, the home-court advantage appears to be alive and well when we're not looking at shooting fouls.

Another interesting development: Home teams aren't seeing a big advantage in blocks per game anymore. In 2010-11, home teams registered 0.81 more blocks per game than the visiting team. This season, the difference between home and away blocks is just 0.43 swats. So while the home/away gap is shrinking in shooting fouls, the same appears to be true of the home/away block disparity.

Evidently, the gap of shooting fouls called is almost even now between home and away. This raises an interesting question: Are 50/50 calls, where it's a borderline block or foul, going toward the visiting team more?

Picture James Harden barreling into DeMarcus Cousins for a driving layup in Sacramento. Lots of contact, but Cousins appears to get the block. The data here suggest that those toss-up calls might be going to the visiting team (in this hypothetical, Houston) more than the home team (Sacramento). But it's hard to say. The margins are just so small at this point. But it's worth digging deeper for another time.

So is this home-court disadvantage a good thing?

Depends where you stand on the issue of predictability. Traditionally, fans of the home team could go to the game expecting to see a victory. Roughly 60 percent of the time, historically, the home team wins. However, that edge is eroding quickly. Take a look at Tuesday night's games: Six fan bases left the arena without a victory. Six games, six home losses. Is that good for the league?

If you ask Houston Rockets general manager Daryl Morey, who has watched his team struggle at home, it's a good thing. From his perspective, the less predictability, the better. Fans like to be surprised. That's why we watch: The drama keeps us coming back. If the home-court advantage made games more predictable, that might mean it's less exciting.

Roughly 60 percent of the time, historically, the home team wins. However, that edge is eroding quickly. Take a look at Tuesday night's games: Six fan bases left the arena without a victory. Six games, six home losses. Is that good for the league?

But there's another school of thought here: Home fans like to feel like they matter. If the home crowd no longer feels like the proverbial sixth man on the floor, will they keep showing up to the game? Why spend your time cheering and jeering if the results show that cheering and jeering aren't as rewarding as they used to be?

Well, because it's fun to see a professional basketball game live and it's still a rush to support your team. You cheer your favorite band at a concert not because it'll make them hit that note a little crisper, but because you're a fan and you want to let them know.

If home-court advantage means little, why fight like mad for seeding throughout an 82-game season?

If you think teams rest their guys leading up to the playoffs now, just wait until they don't think home-court matters. Because what's the point? It's true, the NBA is a business and teams want to get their hands on the gate revenue for that potential seventh game in a series.

But coaches might see it differently. If San Antonio knows that home court isn't worth much anymore, do the Spurs care whether they get in as the fourth seed or the eighth seed? The regular-season mentality might evolve into "just get to the playoffs" as opposed to "get the best seed possible." Is that good or bad for the product in the long run?

Ultimately, if the NBA is cracking down on unfair referee calls, that has to be seen as a good thing. But a move toward fairness might lead to some tricky unforeseen consequences.

We want you to join the conversation. What do you think explains what's going on? And would this be a good thing? We'd love to hear your thoughts. Use the hashtag #HomeCourtDisadvantage and hit us up at @ESPNInsider, @ESPNNBA or @tomhaberstroh on Twitter.