In September 2012, Neil wrote that the NFL playoffs had become more random. And that was three months before Joe Flacco turned into Joe Montana. This year, however, feels like one of the least random playoffs in recent memory. And there’s a good reason for that: it is.

If you know the points spread for a game, you can derive the team’s probability of winning in Excel by using the following formula and typing the spread (with a negative number for the favorite) in cell C2:

=(1-NORMDIST(0.5,-(C2),13.86,TRUE)) + 0.5*(NORMDIST(0.5,-(C2),13.86,TRUE)-NORMDIST(-0.5,-(C2),13.86,TRUE))

Using that formula, the table below shows the winner of each game in the 2013 postseason, sorted in order of ascending pregame win probability. ^[1]All points spread data from Pro-Football-Reference.com. There was only one big upset this year, the Chargers victory in Cincinnati. Conversely, the least surprising outcome was San Diego’s loss in Denver the following week.

Assuming independence, the above odds would imply a 0.28% chance of the 10 games going exactly the way they did. That number doesn’t mean much in the abstract — the chance of any 10 games going in one specific direction is really low — but as it turns out, that is a pretty large number, relatively speaking.

I used this same method to measure how likely or unlikely each set of 11 playoff games has been in each year since 1990. To make an apples-to-apples comparison, I have assumed that Denver wins the Super Bowl and that the Broncos are a 2-point favorite. ^[2]Note to Seattle fans: I have assumed that Denver wins not because they’re the favorite and it would minimize randomness, but because of a personal vandetta I have against the Seahawks the city … Continue reading If that happens, then there would have been a 0.157% chance of the playoffs finishing the way they did (if Seattle wins, that would make the playoffs a bit more random and drop the percentage to 0.124%). The graph below shows the likelihood of the results of each postseason since 1990. The odds are located on the Y-Axis, the year on the X-Axis.

[visualizer id=”17412″]

This jives with what Neil discovered: the results from 2005 to 2011, with one exception, were pretty unlikely. And 2012 followed that same pattern. On the other hand, the 1991 playoffs bears no resemblance to modern playoff football.

That year, Washington was a 7-point favorite in the Super Bowl and won over Buffalo. Two weeks earlier, Washington was a 14-point home favorite against the Lions and Buffalo was a 12-point home favorite against Denver. In the division round, Washington and Buffalo were both double-digit favorites, too. In addition, Denver won as a 3.5-point favorite, while the Detroit/Dallas game was a pick’em. The Wild Card round featured a little suspense, as two road dogs won (Dallas (+3) over Chicago and Atlanta (+6) in New Orleans), while Kansas City (-5) and Houston (-9) won as big favorites. But that was as non-random as the NFL playoffs can get.

What about 2006? Why does that year appear to be not very random? Indianapolis was as a 6.5-point favorite in the Super Bowl over Chicago, and the Bears and Colts both won as three-point favorites two weeks earlier. In the division round, Chicago (-8.5) and New Orleans (-4.5) won as favorites, while Indianapolis (+4) and New England (+5) won as dogs. Yes, even Tom Brady and Peyton Manning can be underdogs. The Wild Card round that year was all chalk: Seattle (-2), Indianapolis (-6.5), Philadelphia (-6) and New England (-9).

Astute readers have probably picked up on something: the spreads have been very low during the 2013 playoffs. In fact, the average favorite has been giving only 4.3 points (including an expected 2 points in the Super Bowl). That’s the lowest since 1992 (4.1). From 1990 to 2012, the average spread was favorite -6.1, so 2013 does represent a pretty significant departure.

One could reasonably argue that the lack of big favorites means the method I’m using understates how predictable this year’s playoffs have been. My formula considers a win by a 3-point favorite to be more random than a win by a 7-point favorite (a defensible position, but a matter of taste). So consider this: only two underdogs have won in the entire postseason. At no point since 1988 has there been a postseason with fewer than two upsets. And there was three or more upsets in every postseason but from 1999 to 2012. So in that regard, 2013 does stand out as a significant outlier; of course, Seattle could still make it three upsets. ^[3]I will note that some outlets had the Chiefs as favorites by gametime, which would mean 2013 already has had three upsets.

Here’s another way in which this year’s playoffs have been unsurprising: for the first time since 1993, there has been only one dog of more than 3 points to pull off a playoff upset. The table below shows, for each postseason since 1990, the probability of the playoffs turning out the way they did (this is the same data from the graph above but in table form), the number of upsets in the playoffs, the number of upsets by teams that were more than 3-point underdogs, the spread of the biggest underdog in the playoffs to win that season, and the average points spread of each game. ^[4]This is from the perspective of the underdog, not the losing team. In other words, it doesn’t matter who won or lost the game when deciding the average points spread.

The average point spread has been very low in 2013, but the favorites are still winning. In that regard, this year has been very similar to the 1992 postseason. Low points spreads indicate uncertainty and are a reflection of parity, so one would think randomness would increase in that environment. But there has only been two upsets in the playoffs, and one of them came by a team with a better record than its opponent.