Many
handicappers have a set of ratings, most often referred to as power ratings,
that gauge the overall strength of each team in comparison to every other team.
They then take the difference in ratings between two teams as the predicted
point differential between the teams if they met on a neutral field. Of course,
teams don't usually meet on a neutral field so points are added to the home
team to compensate for the advantage that most teams have playing at home. The
home field advantage can be a set amount for all teams (such as 2.5 or 3 points
in the NFL and 3.5 to 4 points in college football) or can vary from team to
team depending on their individual variance in their level of play at home and
on the road.
While
the concept of power ratings is rather simple, it is very difficult to come up
with a set of accurate ratings. The problem with most power ratings methods is
that the ratings are generated using some sort of mathematical process based on
the past performance of each team and the level of opposition that they have
faced. An example of this is the Sagarin Ratings seen in USA Today each week.
I've talked to many amateur handicappers that use the Sagarin Ratings to figure
out if the point spread is too high or low on a particular game. What is
important to remember is that the Sagarin Ratings, and any other mathematically
produced set of ratings, explain what has already happened rather than what
will happen. In other words, while it is true that these ratings accurately reflect
the difference in the performance of each team up to that point of the season
they are not a predictive tool to be used to forecast the future performance
level of teams, which is what we are truly interested in as handicappers.
If
beating the point spread were as easy as picking up the Tuesday USA Today,
checking the Sagarin Ratings and making your wagers based on that, then
everyone would be winning and sports books would all be out of business.
Obviously, that is not the case. So, while the Sagarin Ratings can be used to
see how teams have performed up to that point of the season, do not depend on
them to forecast how teams will perform in their next game.
Power
ratings are typically based off of the final scores of games - in football,
there is a lot of 'noise' and 'variance' in scoring, and points are not nearly
as useful for predicting the outcomes of games. Furthermore, power ratings
which reduce every team to a single number ignore the enormous importance of
matchups. If Texas Tech and Georgia Tech have similarly rated offenses, then
you would expect them to fair similarly against a defense that had an average
rating across the board in all defensive metrics. However, against a defense
with an average overall rating, but on a more specific level, with very high
run-defense ratings (allowing 3.1 ypc against opponents who combine for an
adjusted 4.5 ypc) and very bad pass-defense ratings (allowing 9.8 ypa against
opponents who combine for an adjusted 6.4 ypa), you would expect Texas Tech's
pass-heavy offense to fair comparably better than Georgia Tech's run-heavy
offense, even though the two offenses are rated similarly overall. Obviously
analyzing matchups is much deeper and more complex than this, and often gets
into very technical data concerning advantages at individual positions, but
this simple example illustrates the overall concept of how power ratings do not
factor matchups.
