Difference = Point Spread Outcome - Vegas Line
The Point Spread Outcome is computed as Visiting Team Score - Home Team Score since my Vegas Line is referenced in the following way: negative if the home team is favorite and positive if the visiting team is favorite. So, if the difference is HIGHLY POSITIVE it implies that the VISITING team was underrated by the Vegas Line. If the difference is HIGHLY NEGATIVE then the HOME team was underrated. For example, take last year's Super Bowl (Chicago as the 'home team'). The Vegas Line was 6.5 (favoring Indianapolis since it is positive). The actual score was 29-17, hence the actual point spread was 12 and the difference 12-6.5 = 5.5. One can say that the Vegas Line underestimated the visiting team.
I grouped the difference into 7 categories:
- ' < -10' = HOME team underrated by more than 10 points
- ' (-10,-6)' = HOME team underrated by less than 10 points but more than 6
- ' (-6,-2)' = HOME team underrated by less than 6 points but more than 2
- ' (-2,2)' = Vegas got within 2 points of the actual outcome
- ' (2,6)' = VISITING team underrated by less than 6 points but more than 2
- ' (6,10)' = VISITING team underrated by less than 10 points but more than 6
- ' > 10' = VISITING team underrated by more than 10 points
Here is a graph of the percentage of games in each category within each season:
See any trends?
Within each category there isn't any real trends. One thing to notice is that the the two highest categories averaging about 23% are the ones that Vegas misses by more than 10 points (both visiting teams and home teams). Another interesting aspect of this graph is last year's big spike the ' > 10', implying that last year they underestimated the visiting team more than in any other year (One might expect them to correct for this, this year, watch out for underrating the home teams). Come back in a couple of days because I will study the characteristics (if any) of these games where Vegas is missing by more than 10 points, if I find something interesting I will post it.
We know that on average Vegas does well. Since 1992 the average difference is a mere -0.14 with a minimum of -47.5, maximum of 44, and standard deviation of 13. Seems highly volatile to me the fact that on average Vegas is 13 points off the actual outcome. Is Vegas getting better in predicting the point spread? In short, no, the following table shows the mean and standard deviation from 1992 to 2006.
Difference: Point Spread - Vegas Line
Season N Mean Std Dev
1992 204 -0.061 14.412
1993 198 0.15 12.913
1994 204 1.62 12.284
1995 235 0.36 12.601
1996 243 -1.29 12.877
1997 251 -0.23 13.188
1998 251 -0.98 12.185
1999 259 -0.72 13.289
2000 259 -0.61 13.384
2001 259 0.20 13.050
2002 267 -0.24 13.540
2003 267 -1.03 13.470
2004 267 -0.05 12.900
2005 267 -0.89 12.871
2006 266 1.96 13.432