"Throwing Out the Records" in Rivalry Games

A commonly held belief by sports fans is that in rivalry games, the performance of the teams up to that point in the season should be thrown out, since rivalries bring out the best from both teams. These games are generally exciting, so it seems plausible that they are, in fact, closer than expected. So should you really "throw out the records" and assume the teams are closer to evenly matched?

I looked at the past 10 years of results for the following college basketball rivalries:

• North Carolina/Duke
• Kentucky/Louisville
• Cincinnati/Xavier
• Indiana/Purdue
• Villanova/St. Joseph's
• Arizona/UCLA
• Kansas/Missouri (now defunct)
• Syracuse/Georgetown (now defunct)

I first sought to determine if the underdog actually does win more often than Vegas predicts. Converting the spread to an implied win probability for each game resulted in the favorites expected to win 68.37 of the 111 games analyzed, 61.60%. In fact, the favorites won 71/111, 63.96%: slightly more than expected.

But are these games closer than expected, and thus more exciting? The data actually does suggest this: the favorite failed to cover the spread 60.36% of the time. Testing against the null hypothesis that the spreads of these games are 50/50 results in a z-score of 2.22. The corresponding p-value is 0.987: giving fairly high certainty that these rivalry games actually are closer than the betting market predicts.