Post#25 » by Chicago76 » Thu Jan 12, 2012 6:12 am
The resources on the net do a pretty inadequate job of explaining Ortg, Drtg, and WS and Dean Oliver's chapters on it in his book are pretty complex, so it looks like a very simplified nuts and bolts explanation is in order. There are several ways to calculate WS, but they're all variation on what is essentially the same theme. This will only be 95% correct, but it's the 95% you need to understand indiv. Ortg, Drtg, and WS.
The first thing to understand is that WS does not attempt to tell you how many wins a player is responsible for. What it actually does is attempt to allocate credit for wins a team should have achieved, given its offensive and defensive efficiency (or pt differential, since possessions for both teams are roughly equal over a season anyway). This is a pretty small distinction, but an important one, because a team's Ortg and Drtg will set a benchmark for each player.
We get expected team wins from the difference between off and def efficiency in the following formula: Ortg^14/(Ortg^14+Drtg^14) x total games. Assuming team Ortg = 108, Drtg=102, a team would be expected to win 57 games. If league avg. efficiency = 104, then 31 wins come in the form of OWS and 26 come in the form of DWS because the offense outperforms the league avg more than the defense does. Note: you might think the ratio should be 2:1 Off wins to Def wins, but it isn't for a reason we don't need to get into.
These two pools of OWS and DWS (and the corresponding Ortg and Drtg) from the example above are the parameters used to determine individual WS for a player. Individually:
Ortg: is a linear weights measure which tabulates the positive things a player does (made FG, FT, AST, and ORB) against the negative (TOV, missed FG, missed FT) that can be readily obtained in a box score. The objective is to quantify points generated and possessions used for each player to arrive at Ortg for each player. If a player nets to exactly what the team does per poss, then their rating should also be 108. Do less, and you're less than 108. Do more, and you're over 108. Each player is anchored somewhat to the team in such a way that if you possession-weight everyone on a team, this should reconcile to the team Ortg of 108 in our example.
Drtg: a linear weights measure which tabulates all of the positive defensive things a player can do that is captured in the box score: STL, BLK, and DRB. Missed opp FGs and turnovers not from steals are evenly distributed based upon minutes, which is part of the credit problem we'll get into later. Everyone gets a Drtg based upon these "stops" per poss. Just like the offense, if you do more, your rating will be better and so on.
You can then use the formula for team expected wins above on an individual level. You simply need to apply indiv. Ortg and Drtg into the formula just as you would for the team. OWS and DWS are possession-weighted and minutes weighted, respectively vs. team totals to get to this score.
Before we get into the credit issue, it's also important to note that even if a player performs below team avg, some credit will be given. As an example, let's say a player performs 10 pts worse than league average offensively and plays the entire season. On a team level, if a team is 10 pts worse than league average on both ends of the court, they're all-time awful. The would be expected to win 5 games (2.5 on each end of the court) out of 82. So a player responsible for 1/5 of the team's possessions who is 10 pts worse than league avg offensively is still worth 0.5 wins on that end of the court.
This nice thing about WS is that it reconciles well with the team. Now for the bad things, using the player examples earlier in the thread:
-Olajuwon. The value of elite interior defenders is undervalued due to the defensive credit problems. Olajuwon produces value with BLKs, STLs, and DRBs, but a big part of his value is in missed FGs, which are evenly credited among his teammates based upon minutes played. Opposing offenses don't even enter the lane to even attempt shots vs. Olajuwon. Instead, they rely upon lower percentage mid-range and perimeter shots, and Olajuwon doesn't get credit for this. The Rockets went from being the second best D in the league in 93-94 to an avg team (12th) in 94-95. Why? The 3 pt line moved in and their perimeter D wasn't particularly good. Those 2s teams were taking before to avoid Olajuwon were now worth 3 pts. Teams took more of them, and there were longer rebounds on those misses which more frequently became ORBs. On the offensive end, 3s became a more efficient weapon too, making Olajuwon's offensive production relatively less efficient. Olajuwon wasn't worse. The rules changes just made him relatively less valuable. The Rockets adapated their D in subsequent seasons and improved here (along w/ Olajuwon's DWS). Also: in 94-95, Hakeem only played 72 games vs. full seasons for Schrempf, Miller, Barros, which certainly hurt his gross WS number.
-Kidd. True creators are undervalued in WS. They provide easy shots for teammates, and the incremental production is credited in the form of efficiency to the scorer. Sure, there is an assist component, but all assists aren't equal. A guy who can demand double teams on penetration before dishing has value that a more run of the mill PG doesn't have. The same goes for scorers. Speaking of which, Kidd was horribly inefficient as a scorer. This killed his Ortg and WS. Also: great defenders who don't necessarily generate a ton of steals or blocks will be undervalued. Kidd did get a lot of steals but hardly any blocked shots.
-Very efficient third and below option scorers are almost always overrated. These players are efficient because they're good at knocking down shots the defense concedes to take away the primary scoring threat. In a perfect world, some of this credit would be given to the lead offensive players, but how much? There is no clean way to allocate. This is why the Jon Barry's of the world (and Detlef Schrempfs on more balanced teams) can generate huge WS. The distinction here isn't so much efficiency as where in the offensive pecking order a player is. Steve Kerr got wide open looks and was efficient in hitting them thanks to Jordan, Pippen, Kukoc, and the need to defend easy buckets inside. He was the net beneficiary of great scorers/creators. Reggie Miller on the other hand was deadly efficient as a primary scorer. He played off Smits probably about as much as Smits played off him, but defenders weren't exactly preoccupied with the Davises, Derrick McKey, the shooting touch of Mark Jackson, etc. Miller's deep threat at volume benefitted team spacing and offense rather than the offense benefitting Miller.
Hopefully this helps someone conceptually. The stat isn't flawed. It's a gross stat (as opposed to PER, which is a rate stat). As far as gross stats go, it is a very useful one. Just compare WS to another gross stat (like total points scored in a season) and tell me which one does a better job of ranking players. Like any statistic, you just need to understand what it can and can not measure rather than discarding it indiscriminately.