APBRmetrics

[gabefarkas]

[mtamada]

[mtamada]

[back2newbelf]

http://www.sportsbook.com/livesports/indexmember.php?sportsname=basketball
vegas now with predictions for all the teams.
The Suns and Nuggets are predicted to win substantially less. Miami at 64.5 right now. T-Wolves lowest at 23.5

[schtevie]

Dang. I should have gotten in on the action at 63.5...

Mike, I think that there is value in being more precise when employing the term "regression to the mean" (RTTM). There are two distinct types of phenomena generating the results we see with NBA, league-wide win estimates across seasons, but it is only the first phenomenon that should be meaningfully associated with the term.

The first is statistical, the second for the lack of a better term, is fundamental or model-based (and with this, the zero-sum constraint on the estimation exercise is an important factor).

What I am referring to as "statistical" is the RTTM you observe when, on average, the kids of tall parents don't grow up to be quite as tall. The counterpart in the NBA is when a team because of truly transient good fortune generates wins in excess of what its "genes" would predict, and the following year (all else equal, which it never is) tends to regress to its expected ability.

What I am calling fundamental or model-based phenomena are the effects of all factors that would fit in the "true" NBA production function (that what G*d would reveal, were she so inclined).

One thorny unknown of particular importance in this regard is the precise effect of aging and injuries. But the general story is known and agreed upon. Younger players improve as their skills and knowledge of the game improve. Past their peak, athleticism deteriorates and the propensity for injury increases.

And how does this class of variables (and their variability) influence apparent RTTM? Well, in the ways we are used to expecting, but this result has nothing to do with the "laws of statistics". Apparent RTTM obtains because of the additional fact that better teams tend to have more minutes played by players at their peak, hence more prone to aging and injury effects, and also because the NBA is a zero-sum league (i.e. the games "unexpectedly" lost due to injury and aging, disproportionately suffered by the better teams, pad the win totals of the below-average younger, healthier teams)

As a thought experiment, imagine we lived in an alternate universe where human physiology was different, such that the young rather than the old tended to be injury-prone, and the league had a really hard salary cap instead of a soft one. Much of the apparent RTTM would vanish.

In light of these arguments, we can (kind of) interpret the recent Celtics' history that was mentioned. The last three years saw win totals of 66, 62, and 50 games.

The first issue is: to what win total should the Celtics have rolled in the first year of the Big 3? (The same question as for the Heat this year.) And the answer is, of course, that I don't really know the precise answer. Nobody does, but 66 is probably pretty close to what the "one true model" (approximated best by APM) would have predicted. KG was out for a stretch and played relatively poorly, just prior and after, but, then again, maybe Paul Pierce was playing "above his age" during the season. Etc.

But that emphasizes the main point: the expected value of this production function for the year in question is what matters for a prediction, and no adjustment for RTTM should be made ex post. (If one ends up requiring ad hoc adjustments, it necessarily implies that the model being used to generate predictions is incomplete and giving systematically biased results.)

What then of the following two years? All that can be said is that the trajectory is entirely consistent with fundamentals, but beyond that I have no firm idea as to how they should have been expected to perform. The 62 games seems about right, and last year, my sense is that they endured quite a few "excess" injuries. But I don't have any great confidence about that statement. And what to expect this year? I wish I had a clue. My guess is that they will "regress" to something above 50 wins. And that is the point.

[Crow]

The Heat line might move again depending on whether Wade & James are considered ready & 100% closer to or on opening day or are still that way 2-4-8 weeks in, if they adjust the line and still take bets. At least in the abstract it should.

[Rhuidean]

Schtevie: You've build a model that tells you X. You also have prior knowledge Y about the situation. Unless you have the utmost confidence in your model X and its results, then it makes sense to consider Y also.

From what I've seen (certain ad-hoc down-weightings of APM/SPM values that people used for LBJ, Wade, and Bosh, weighted APM/SPM values that lead to something close to 100% wins), it doesn't seem as if we should have utmost confidence in most of the models just yet. So incorporating prior knowledge makes some sense.

[back2newbelf]

Can anyone explain me what this vegas line means?
Atlanta Hawks Over 46.5 (-115) Under 46.5 (-105)
specifically the "-115" and "-105".
Obviously they didn't like their initial projection anymore and changed it a bit, but in what direction?

Also, I want to take back my earlier comments about the Knicks. They added more players than I remembered and Stoudemire looks good (at least in pre season)

[haralabob]

[HoopStudies]

Am I mistaken or did this thread not actually have anyone's actual predictions in it?
_________________
Dean Oliver
Author, Basketball on Paper
The postings are my own & don't necess represent positions, strategies or opinions of employers.

[back2newbelf]

[Ed Küpfer]

The Painted Area compiled forecasts from Vegas, John Hollinger, Kevin Pelton, and Kelly Dwyer.

[Kevin Pelton]

[back2newbelf]

[BobboFitos]