Do 12 Seeds Really Upset 5 Seeds More Often Than Expected?

The 12 seed over a 5 seed upset pick is a popular one in bracket pools everywhere. So much so that the value that may lie there is eroded now by its popularity, or so it seems. But do 12 seeds really pull off the upset to an abnormal degree? At least when compared to 11 over 6 and 13 over 4 (relative to their respective seeds)?

If there is a significant difference, the best explanation I've heard (via Mark Titus, @clubtrillion) is that the 12 seed is usually an auto bid mid-major that got hot and won their conference tournament to play their way into the field. Meanwhile, the 5 seed is often a Power 5 team that slumped towards the end of the season and fell down into the spot. Examples of this this year include Ohio St/South Dakota St and Clemson/New Mexico St, where Ohio St finished the regular season 2-3 and Clemson ended the year 3-5 (and their counterparts won their conference tournaments).

I used all first round tournament results since 1985 to determine the trend, and the 12/5 matchup is indeed an outlier:


The 12 over the 5 is actually on par with the 11 over the 6, and very close to the 10 over the 7. Using the linear regression illustrated above by the trendline, we should expect the 12 to beat the 5 only 29.12% of the time. Testing against this null hypothesis generates a z-score of 4.66 over 1,056 games, so we can determine with a very high level of certainty (99.9998%) that this isn't a result of random chance: the 12 seeds do win more often than they're supposed to.