By Brian Burke
In week 1 the average HFA was 3.2 net points, and in week 2 it was 8.7 net points. As I write, Week 3's average is -1.9 net points through 14 games. Week 1 was obviously well within the long term trend of 2.5 net points for the home team, and Week 3 tilted slightly toward the visitors. But week 2 showed a large advantage for home teams. We know there is week-to-week variance in net scores for home teams, but how far from ordinary is 8.7 net points for the home team in a single week?
Since 2000 the standard deviation for weekly average HFA is 4.0 points, and the average is 2.4 points, which means week 2's 8.7 is not 2 standard deviations from the mean (p=0.40). Week 2 featured only the 15th largest HFA in the past 12+ years. The regular refs were on the field for the 14 weeks with larger HFAs.
The chart below shows the distribution of weekly average HFA in net points for the home team.
[Incidentally, the oddest week in the data was week 7 in 2009. Visiting teams beat home teams by an average of 14.3 points, or about 30-17.]
This is not to say that the replacement refs are not influenced by home crowds or that they're doing a good or bad job. It's just that we can't tell by looking at net point HFA.
The source of HFA is mysterious. Research into crowd effects, officiating bias, and travel are inconclusive. There are sports which travel is not a factor that have HFA, and there are sports that generally have no crowd that also have a HFA. I have my own theory about where much of HFA comes from, but that's not to say that other factors don't enhance or mitigate it. Regardless of the cause of HFA, we'll need a lot more than a few weeks of games to be confident that the replacement refs are affecting it.