NFL Playoff Rematches: Is History A Good Teacher?

Tomorrow’s conference championships are NFL playoff rematches of regular season battles. The Seattle Seahawks and New England Patriots both scored decisive wins over the Green Bay Packers and Indianapolis Colts, respectively. Historically, what do earlier duels tell us about results of subsequent rematches against the spread, and does margin of victory matter? Konstantin Medvedovsky digs into decades worth of data and explains.

As of this writing, the Patriots are favored over the Colts in the AFC Championship Game by 6.5 points. This predicted margin of victory falls in line with what most computer metrics would expect.[quote align=’right’]Simple Rating System is a simple system based on a team’s margin of victory and strength of schedule to date. It calculates a team’s margin of victory, and then adds in an adjustment for the average margin of victory of the teams played so far. The version reported by Pro-Football-Reference uses regular season SRS numbers, which does not credit the Colts for their extremely impressive playoff performance to date. My version adds in the playoffs and weights more recent games more heavily.[/quote] For example, my version of Simple Rating System (“SRS”) says the Patriots should be favored by approximately 7.1 points. Football Outsiders expects a closer game, putting the line at 5.7 points. However, both these systems largely ignore the fact that the teams played earlier this year, with the Patriots winning comfortably on the road, 42-20.

Does this matter? Do playoff rematches resemble their regular season counterparts? Let’s take a quick look through NFL history to see what can be gleaned.

Our database of games since 1978-2013 (courtesy of Pro-Football-Reference.com), reveals 200 rematches. Overall, the data looks like this:

Note: ATS numbers do not sum to 100% because of pushes. “Delta” represents the difference between the actual margin of victory and the spread.

The average winning team in the regular season won by 11.12 points. In the subsequent rematches (across all rounds of the playoffs), the regular season winner won again 65.5% of the time, by an average of 5.46 points. This slightly beat Vegas expectations, but only by 0.43 points. Overall, basically what we’d expect. The regular season winner was the better team, and Vegas more or less correctly anticipated the result, with the prior winners only slightly beating the spread (Note: Because of the vigorish, which is a sports book’s minimum commission on any wager, a bettor needs to win approximately 52.4% of the time to break even on a bet.)

However, the Patriots’ regular season victory was a blowout, as they won by three scores despite being three point underdogs. What if we consider just the blowouts? 49 of these games could be classified as “blowouts”, meaning the winning team won by three scores or more, or 17+ points. In the subsequent rematches:

All of a sudden, a sizable effect emerges. While Vegas expected the regular season winner to prevail by an average of 7.05 points in the subsequent rematches, in reality, they won by 10.22 points. Additionally, the blowout regular season winners went an impressive 57% against the spread, enough to make a healthy living over the long term. Overall, Vegas underestimated the predictive importance of having won in blowout fashion previously by a sizable 3.17 points.

One final pivot point: the Patriots didn’t just win by 22 points, they won by 22 points on the road. At the risk of reducing the sample size further (and thus increasing the importance of random variation), here’s what the data looks like for 17+ point winners on the road:

The measured effect becomes even stronger. Vegas is underestimating road blowout winners by 5 points, and they are absolutely crushing the opposition against the spread. It’s important to reiterate that this is only 25 games, of course, but either way it bodes quite well for the Patriots this Sunday.

Of course, determining the degree to which those 3.17 or 5.08 points represent luck/variance vs. a “matchup” effect is impossible to determine, but it is at least suggestive that prior blowout winners are in fact being underrated in their subsequent playoff matchups. Given the stylistic similarity of the Patriots’ victory this year to their 42-22 win in last year’s playoffs over the Colts, my instinct is to believe there is something “real” to these numbers ‒ at least in the case of Sunday’s AFC title game.

Follow Konstantin on Twitter @kmedved.

Konstantin Medvedovsky analyzes data, looking at home field advantage and weather, or ifearly-season results are predictive.