Sunday, April 17, 2011

The Prophet Dick Vitale

We'll return for a moment to the issue of home court advantage.

RPI normally adjusts for home court advantage by weighting home games differently than away games and we looked at adjusting by points.  In both cases we found that adjusting for HCA actually worsened the performance of our predictor.

A radically different approach to dealing with HCA was suggested by a comment from the fount of all basketball knowledge, Dick Vitale.  During the 2011 season, commenting on St. John's -- a team that was hard to beat at home but weak on the road -- he said:

    "They're a different team at home, baybee!"

This keen observation suggests that we could account for the HCA by treating each team as two different teams: a "St. John's at home" team and a "St. John's on the road team".  To do this, we calculate a "Home RPI" based upon the team's winning percentage at home, the team's opponents' winning percentage on the road, and the team's opponents' opponents' winning percentage at home.  The "Away RPI" is calculated in the opposite manner.

Given that home teams win 2/3 of the games, you might expect teams to have better Home RPIs than Away RPIs.  In fact, just the opposite happens.  Recall that the formula for combining the parts of the RPI is:

    RPI = (WP * 0.25) + (OWP * 0.50) + (OOWP * 0.25)

The OWP is the largest part of this calculation, and that's largest when we're looking at the opponents' home records, i.e., when we are calculating the "Away RPI".  So in general, a team's "Away RPI" tends to be higher than its "Home RPI".

For the 2011 season, the top five away teams were:

  Team    ARPI  
San Diego St.0.715
Ohio St.0.703

Reviewing these teams' road records, this looks fairly accurate.

Unfortunately, breaking RPI down into home and away doesn't seem to have a lot of predictive value.  Using the home team's "Home RPI" and the away team's "Away RPI", we get this performance:

  Predictor    % Correct    MOV Error  
RPI (unweighted)74.6%11.53
RPI (home-away)69.0%12.5

...considerably worse than our previous best performance.  So Dickie V's advice turns out to be worthless.  (But we shall return to consider Mr. Vitale's wisdom again in the near future, where it may prove more valuable.)

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