For those who may not have seen CHODR last year, or those who would
like another look, here's some explanation of the rating. (This
week's ratings were included in a separate post)
CHODR - The College Hockey Offensive / Defensive Ratings
(pronounced "chowder")
1. What is CHODR?
CHODR assigns Offensive, Defensive, and an Overall rating to each of
the 44 Division I Men's Ice Hockey teams. (Massachusetts is not
eligible for the NCAA tournament and is not included in the ratings)
2. How are the ratings determined?
CHODR places equal emphasis on offense and defense and provides an
overall rating for each team based on its offense-defense
combination. A team's Offensive rating is based on the goals it has
scored, with an adjustment for the quality of the defenses it has
faced to score those goals. Similarily, the Defensive rating uses
goals allowed and adjusts for the offensive ratings of the opponents.
The philosophy behind the adjustment calculations is similar to the
least squares optimization that has been used by TCHCR. The Overall
rating is just the difference between a team's Offensive and
Defensive ratings. A more technical explanation of the math is
available for those who are interested.
3. How can the ratings be interpreted?
(a) An offensive/defensive rating represents the goal scoring/
allowing rate (average goals per game) which would be predicted
against a mythical "average" team on neutral ice. An Team with an
Offensive Rating of 6 and a Defensive Rating of 2 would therefore be
predicted to beat an "average" team on neutral ice by the score of 6
to 2.
(b) A positive/negative Overall rating indicates that a team has
more/less overall scoring potential than the "average" team.
(c) We can rank teams according to their overall ratings or provide
separate rankings to reflect offensive or defensive prowess. The
better teams have the higher overall ratings, while the better
offenses have higher offensive ratings and the better defenses have
lower defensive ratings.
4. Comparing individual teams.
One nice feature of this system is that it allows comparisons between
teams on an easily understood numeric basis. Basically, the
difference in overall ratings between two teams represents the
predicted average goal differential if they were to play lots of
games on neutral ice.
Example: Mass-Lowell (1.24 Overall) vs. Colorado College (0.74
Overall) would predict an average goal differential of about 1.24 -
0.74 = 0.50 or 1/2 goal in Mass-Lowell's favor.
5. Predicting future scores.
Another bonus of CHODR is that it may be used to forecast future
scores. The relevant formula is
Team A's predicted goals = A's Offensive Rating
+ B's Defensive Rating
- Average Offensive/Defensive Rating
+/- Home Ice Advantage
Team B's predicted goals = B's Offensive Rating
+ A's Defensive Rating
- Average Offensive/Defensive Rating
+/- Home Ice Advantage
The Average Rating is subtracted from each team's predicted score
because the average is already included in each teams rating, so it
ends up getting included twice when the ratings are summed. The Home
Ice Advantage value gets added to the home team's predicted score and
subtracted from the away team's predicted score. No adjustment is
made for a game on neutral ice. The Home Ice Advantage value
(currently +/- 0.226 goals) is based on the amount by which home
teams hove outscored their oppposition to the current point in the
season.
EXAMPLE: Let's try RPI ( Off = 4.35, Def = 3.57 ) at
Maine ( Off = 4.97, Def = 2.47 ):
RPI's predicted score = ( 4.35 + 2.47 ) - 3.88 - 0.226 = 2.71
Maine's predicted score = ( 4.97 + 3.57 ) - 3.88 + 0.226 = 4.89
Although it may seem awkward to predict non-integral scores, this
number actually represents the expected average score for the given
team if they played many games under the given conditions (same
opponent and same location). Of course, in any one game, the actual
result may or may not be close to this average. As we all know,
upsets have been known to occur from time to time.
6. Notes:
(a) We do NOT use any information about who wins or loses a game in
generating the ratings. We are intending only to rate a team's
ability to score and keep the other team from scoring. This provides
a direct contrast to TCHCR.
(b) Goals scored in overtime are ignored when generating ratings.
We'd also like to eliminate empty net goals, but getting reliable
data on those is usually not possible. Thus our ratings reflect the
regulation time goal scoring ability.
(c) As in TCHCR and RPICH, only games between two Division I teams
are counted. No prior information is used in the rating, i.e. all
teams are equal at the beginning of the season.
(d) Because of the way ratings are calculated, scoring 5 goals
against Lake Superior (Defensive = 2.46) will help your Offensive
ratings a lot more than doing the same to Air Force (Defensive =
6.14). Likewise, allowing 6 goals against Michigan (Offensive =
5.68) won't hurt your Defensive ratings as much as allowing the same
number to Cornell (Offensive = 2.49). Thus there is an implict
strength of schedule concept here just as in TCHCR.
(e) The estimated ratings are based on a fairly small number of games
so far and will tend to exhibit some variability until later in the
season. Inter-league play will have a large effect on these ratings
just as it does in TCHCR.
(f) We do not intend at all that CHODR should "compete" with TCHCR.
Rather we hope to supply a different perspective to help spark
discussion. The two rating systems complement each other nicely,
since TCHCR concerns itself only with the outcome of the game
(win/loss/tie), while CHODR is concerned only with the score,
regardless of the game outcome.
This rating is a joint effort between both of us. Questions,
comments, and/or suggestions about CHODR are welcomed and encouraged.
Please don't post questions to the list, but mail to us directly. If
possible, please send mail to both of us when inquiring about the
rating.
Timothy J. Danehy [log in to unmask]
Robin Lock [log in to unmask]
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