Note: This article updated after the games of March 6.
Michael Jordan's best player efficiency rating (PER) was 31.71. LeBron James peaked at 31.67. Wilt Chamberlain is estimated to have posted the all-time record of 31.82 in the 1962-63 season.
And now Stephen Curry is poised to top them all, with the best PER we've ever seen.
But does that really mean Curry is having the greatest season of all time? Let's take a look.
Curry's player efficiency rating stands at 32.3 with a quarter of the season to go. He has already broken his own record for 3-pointers in a season. He's the reigning MVP and on course to be the first-ever unanimous MVP.
And, oh yeah, his team is 55-6. The defending champion Golden State Warriors stand just 18 wins from setting the record for most in a season. It's a heck of a résumé.
But beyond PER, we should examine more advanced stats to determine whether Curry is truly playing better than anyone before.
Best scoring season ever?
Let's start by isolating Curry's best skill, his scoring. From a statistical standpoint, players help their teams as scorers in two ways: by creating scoring opportunities and taking advantage of them efficiently.
We measure these two skills with usage rate (the number of plays that a player uses, measured as the percentage of individual finishes with a shot attempt, trip to the free throw line or turnover) and true shooting percentage (points scored per shot attempt or trip to the free throw line).
These skills tend to be in conflict. If a player uses more plays, his efficiency tends to go down, naturally. More shots usually means more bad shots. But that's not the case for Curry -- he can shoot more without losing efficiency.
If you graph these two skills for players since 1977-78, dividing true shooting percentage by league average to reflect changes in typical scoring efficiency over time (TS+), you find that there's typically a limit on how good players can be at both skills simultaneously. John Hollinger was one of the first to write about this effect, and Evan Zamir studied this frontier in 2012.
Since then, Kevin Durant and LeBron James have nudged past the old threshold.
Curry's 2015-16, by contrast, is obliterating it -- his new standard is represented in red in this chart.
I've created "adjusted true shooting percentage," which combines the two skills of creating and finishing shot opportunities into a single stat.
Again, dividing this by the league's typical efficiency (ATS+), Curry is having a record-setting season:
By increasing his 3-point volume without sacrificing accuracy, Curry has defied the usual inverse relationship between usage and true shooting percentage.
He's using plays at a career-high rate while also scoring more efficiently than ever before -- more efficiently than any high scorer ever. No player before with at least 1,500 minutes has surpassed a .680 TS+ with a usage rate higher than league average (20 percent).
So, yes, this is shaping up as the best scoring season ever, considering Curry's efficient yet prolific ability to put points on the board.
Best overall offensive season ever?
Focusing solely on scoring sells Curry's offensive ability short. You'll note something about the other players on the above list, as I did when making the case for Curry as MVP last year: they're all frontcourt players.
Curry already had posted the best combination of scoring and efficiency ever for a guard in 2014-15, and he has blown that away this season.
As a result, Curry's lead grows when we account for his playmaking. The only player on the list with a better assist rate than Curry this season (8.4 per 100 possessions) is ... Curry last season.
Then there's the matter of how Curry's shooting opens up the floor for his teammates because of the defensive attention he draws, a concept I call "gravity."
When Tom Haberstroh used SportVU data to quantify gravity, Curry had the highest respect rating from defenses in the league in 2013-14. That's only grown with Curry now a threat to pull up anywhere on his team's side of the midcourt stripe. Just by setting foot on the court, Curry makes life easier for his teammates.
When you add up all those factors, the question isn't really whether Curry is having the best offensive season ever, so much as whether anyone else is even close.
That question is easy to answer -- no one is even close -- when looking at box plus-minus (BPM). Based on similar techniques as those behind ESPN's real plus-minus (RPM), BPM was created by Daniel Myers and is tracked by Basketball-Reference.com. It weighs box-score stats by how well they predict a player's team impact as measured by adjusted plus-minus.
In terms of offensive impact as estimated by BPM, Curry rates 13.1 points per 100 possessions better than an average player this season -- which is 3.3 points better than the next-best rating on record, Michael Jordan's 1987-88 (MJ was at 9.8 in offensive BPM). The gap between Curry and Jordan is larger than the gap between Jordan and the 60th-best season in offensive BPM.
Best season ever?
Given that PER tends to heavily weigh offensive statistics, it's no surprise Curry is posting the best PER ever. That noted, even metrics that give a fuller view of offense and defense have Curry at the top.
His 13.2 total BPM would surpass LeBron James' 2008-09 (plus-13.0) for the best season since team turnovers were tracked in 1973-74.
And Curry's .337 win shares per 48 minutes would be the best in the same era. Only Kareem Abdul-Jabbar, with .340 in 1971-72, has been better, using Basketball-Reference's estimated stats.
The lone dissenter on a per-minute basis is RPM. Curry's plus-10.1 RPM is best in the NBA this season but ranks just sixth in the RPM database dating to 2000-01, trailing two James seasons, two by Kevin Garnett and one from Chris Paul.
Still, the consensus favors Curry as long as we're looking at per-minute stats -- which means we can say with some confidence that Curry is playing at a higher level than anyone in the past four decades.
When we factor in minutes played to get Curry's total contributions, the picture becomes a little murkier. For instance, my wins above replacement player (WARP) metric leaves Curry out of the all-time top 10, simply because he hasn't played the same minutes as the players above him.
Once again, even with my metrics, Curry is the best on a per-minute basis (my version of this metric is called "win percentage"). But because he plays just 34.0 minutes per game, Curry comes in 13th overall in WARP.
WARP rewards players for outperforming a replacement-level player (essentially, a free agent signed for the minimum salary) while on the court, so playing time is a key factor. Curry has averaged nearly three minutes fewer per game than anyone ahead of him in WARP.
Jordan, by playing more than 40 minutes per game, at a very high level, holds the WARP record for a single season. His 1988-89 season, in which he finished second in MVP voting and his Bulls won 47 games and lost in the conference finals, is on the short list of greatest statistical seasons by an individual. MJ posted two other seasons almost as good, both also before winning his first title.
The verdict
So just what constitutes the best season ever? Should we focus on total value, as stats like WARP are trying to measure, or is it about level of play?
In this case, I'm tempted to lean toward the latter definition. Stamina is not the issue with Curry. Rather, Warriors coaches Steve Kerr and Luke Walton are choosing not to play Curry more minutes because they don't need to during the regular season.
He has famously sat out 16 fourth quarters of Golden State blowouts this season -- already more than James sat out in 2008-09 (14).
Given the Warriors' 55-5 record, it's hard to imagine Curry could be doing much more to help the team win. So based on the season so far, I'm willing to give Curry the nod, at least in the advanced-stats era.
In the early years of the NBA, key stats like blocks and steals (let alone turnovers) weren't tracked. So when it comes to comparing Curry to Chamberlain, Bill Russell and Oscar Robertson, it's a little trickier to say whether Curry's season is really the best ever.
But based on the way league quality of play has improved over time, I believe nobody has been better than Curry.