My Highly Plausible Prediction model this year projected the Celtics at 49 wins, a number above their 40 win performance last year, the initial Westgate line on them at 42.5 wins and the general consensus. I have been asked a couple of times whether I "buy" the model's number or not, and the answer to that is, of course, both yes and no.
The ambiguous answer comes from two parts. First, is my grasp of the model's strengths and weaknesses along with last year's model performance, which I will discuss below. Second, is the nature of what I am trying to do in making my predictions in terms of contributing an independent estimate to the general knowledge base.
The second point is interesting in this way;
If you want to make the most accurate prediction it is best to give significant weight to the wisdom of crowds, but the wisdom of crowds doesn't exist if no one is contributing their independent estimates.
If everyone tries to free ride on the crowd, there is no wisdom generated. Similarly in the separate but related economic idea of the efficient market hypothesis, it is sometimes said that it can only ever be approximately true because the market needs traders attempting to beat the market to make the market function. Nate Silver has also pointed to pollsters tailoring their polls to fit the consensus as a factor weakening the performance of polling averages in recent elections.
So, in markets, election predictions or sports predictions individual predictions are made better by pooling information from the consensus, but the consensus itself is made weaker if that behavior is too widely adopted. In this case my model is meant as an independent estimate, likely a best guess estimate would weigh it with something like the ESPN forecast that aggregates the opinions of ESPN writers and bloggers, but I think it is a strong and plausible estimate on its own.
The projection model itself last year was among the best publicly available analytic projections, for whatever that is worth. More concretely, the median error was 4 games and the average was 5.9 games per team, the larger average miss compared to median indicates there were a couple of outsized misses, like underestimating Atlanta and overestimating Minnesota and OKC, that pulled average number up. But in either case that is a good amount of expected error.
Celtics Specific Issues
But there are a couple of items specific to the 2016 Celtics prediction that could cut in in both directions. The first is schedule, according to my ratings the Celtics have the second easiest opponent schedule in the league with their opponent's average win estimate at 39.5 wins. Being in the Atlantic division is helpful, playing the Nets, Knicks and Sixers four times a piece is likely to pad the wins at least a touch.
A good deal has been made of the Celtics' "mish-mash" roster and lack of talent separation among players this off season. The lack of certain skills and the possibility of too much skill overlap, not enough shot creation too many defensive oriented guards and wings, for example, could be a source of systematic error. This is reduced somewhat by enforcing rotation minutes that balance out positions, including, for example, increasing minutes for Evan Turner, a versatile point wing but also the lowest rated Boston rotation player. However, fit and overlap remain unknown potential sources of error.
The lack of separation is a double edged sword. Mentally assessing team strength it's easy to default to something like a playoff rotation model with all starters available and a shortened rotation for the bench. However, only nine players started all 82 games last year, and only 76 players, about half of the available opening day starting positions started at least 80% of the games. In that high frequency starter set the median minutes played was about 2,400 minutes, about 60% of the available minutes. In the next group starting over half the games but less than 80%, 52 players per basketball reference, the median playing time was about 2,000 minutes, or about only 50% of available playing time. In the regular season, at least, depth matters, though it is hard to predict before the season who it will matter for the most.
Lastly. the Celtics have a young group, with only Amir Johnson and David Lee on the wrong side of the aging curve. Their net aging curve, the amount of improvement or decline expected across the team weighted by minutes played is +1.99 wins. Some young players clearly won't improve, the age curve is a generalized trend, but having more young players gives the team more rolls of the dice on one of them having a breakout season.
Ultimately, I in terms of what I believe the Celitcs are most likely to do in the upcoming season, I would take the over on the Westgate line, but probably not the over on the Highly Plausible model's projection.
