Posted by Neil Paine on September 28, 2009
I know we haven't talked about Dean Oliver's Four (Eight?) Factors here in a while, but that hasn't been deliberate. I actually like the 4 factor methodology for evaluating teams' strengths and weaknesses, although there's a quite a gordian knot to deal with when you start trying to link team factors to their respective metrics for individual players. Hmm... maybe that's the reason why I haven't invested so heavily in them recently, because we've been all about trying to establish expectations for teams in 2010 based on their current rosters, and last year's 4 factor data wouldn't help you get very far in that direction. But it occurs to me that another way to look at team trends is to see which stats are historically sustainable from year to year, and which aren't. So, with that in mind, here are year-to-year correlations for each of the 4 factors, on offense and defense, since 1973-74 (the first year the NBA kept turnovers, offensive rebounds, etc. at the team level):
Rebounding ability seems to be the most consistent aspect of the four factors from season to season, followed by the ability to prevent and force turnovers. Interesting how the non-shooting areas (Reb% and TO%) are more consistent than the ones that deal either completely (eFG%) or partially (FTr) with putting the ball in the basket... I have no numbers to back this up, but I always felt like shooting was by far the place in the game where luck had the biggest impact. You never hear about "streak rebounders" or "streak ballhandlers", but streak shooters have been in the lexicon practically as long as the game has been around. There's something about the act of tossing a ball into a stationary target 10 feet off the ground that lends itself to that kind of randomness, I suppose.
Now, free throw rate is a strange case because it combines several different aspects of the game. Shooting accuracy is obviously a large part, and drawing fouls is a function of your own offensive aggressiveness (rarely do they call fouls on 20-foot jump shots), but you also have to rely on the officials to give you the calls when you do attack the basket. So here are the year-to-year correlations of the two separate metrics that make up free throw rate:
You might think that free throw percentage would stay relatively constant year-to-year, since the conditions never change and the defense can't bother you when you're shooting... However, chalk this up as more evidence that shooting a basketball accurately is very tough to do on a consistent basis, even if you're all alone in a gym. Finally, I wanted to know which field-goal shooting numbers varied the most, 2-pointers or 3-pointers, but I wanted to filter out the early years of the 3-point era (when few players and teams seemed to understand the value of using the new shot) as well as the brief 22-foot arc in the mid-1990s. So these numbers are only going to be on seasons after 1996-97, the final year before the NBA went back to a 23'9" arc.
So, as you probably expected, it's a lot easier to shoot with consistency the closer you get to the basket, but even I'm surprised at the low year-to-year correlation we see for opposing 3-point %. One team that was unusually fortunate in that regard was Cleveland, who allowed a league-low 33.3% shooting from the arc, almost a full percentage point better than the 2nd-place Magic; conversely, the Sacramento Kings allowed a staggering 40.5% mark from downtown, which helps explain some of why they had the worst defense in the NBA, more than 1 pt/100 poss. worse than Washington, the 2nd-worst D. Here are more teams who performances in certain categories are sure to regress -- and progress -- towards the mean:
Finally, keep in mind that even though these are lists of teams that got unusually "lucky" or "unlucky" last season, regressing these numbers to the mean will result in only relatively small improvements/declines in W-L%, because free throws & 3-pointers only account for roughly 40% of all points and 30% of all possessions. But even so, those small changes can be very important to teams that are close to playoff contention.