Does last season matter? - Follow Up
/I wrote a few weeks ago about the weak predictive information contained in a team's previous season of data. When trying to predict a team's goal differential in the second 17 games of a season, it was the first 17 games of that same season that did the job. The previous season's data was largely unhelpful. @sea_soc tweeted me the following:
https://twitter.com/sea_soc/status/406507942179905537
Ask, and you shall receive. Here's the weird shit I found when trying to project a seasons second-half goal differential:
| Stat | Coef. | P-Value |
| Intercept | -33.6 | 0.86% |
| AttemptDiff (first 17) | 0.1 | 0.00% |
| Finish Diff (first 17) | 90.6 | 0.12% |
| Attempt Diff (first 17 last season) | 0.1 | 2.88% |
| Attempt Diff (second 17 last season | 0.0 | 20.00% |
| Finish Diff (first 17 last season) | 115.0 | 7.08% |
| Finish Diff (second 17 last season) | -23.5 | 28.81% |
| Home Games Left | 4.0 | 0.81% |
Translation: Strangely, it's the first part of the previous season that is the better predictor of future performance. Not the second part of last season, which actually happened more recently. In fact, information from the second part of each team's previous season produced negative coefficients (negative relationships). Weird.
Now let's change the response variable slightly to be a team's goal differential from its first 17 games. Which does better at predicting, last season's first half or last season's second half?
Neither. In fact, there was nothing that came close to predicting the first halves of 2012 and 2013.
| Stat | Coef. | P-value |
| Intercept | 18.9 | 20.3% |
| Finish Diff (first 17 last season) | -5.5 | 94.5% |
| Finish Diff (second 17 last season) | 5.9 | 60.9% |
| Attempt Diff (first 17 last season) | 0.01 | 26.6% |
| Attempt Diff (second 17 last season) | 0.04 | 32.5% |
| Home Games (first 17 this season) | -2.2 | 20.3% |
With such small sample sizes, it could be there is just something really weird about the first halves, especially 2013. I say "especially 2013" because 2011 and 2012's first halves seemed to do a fair job of projecting the next season's second halves, so it's 2013 that seems screwy. Portland and Seattle performed opposite of what would have been expected for each, for example, while D.C. United and Montreal did the same confusing switcheroo in the Eastern Conference to kick off the 2013 campaign. So it could have just been weird randomness.
In the end, I'm quite certain of one thing, and that's that I'm still confused.