Chiefs-Broncos games are closer than you might expect

The Chiefs take on the Broncos tonight for the biggest rivalry game of the season. After an impressive win in week one the Chiefs are 3-point favorites, although since they are playing at home, that typically means sports books think the teams are about equal right now.

But the teams have not always been so evenly matched. This heated rivalry has ebbed and flowed over the years with the success of each team. Since the NFL has used a 16-game schedule, the most wins the teams have combined for in a single season was 25, in 1997.

Perhaps not coincidentally the match-ups in 1997 are considered two of the best ever, with each game decided by four points or less and one game coming down to a 54-yard game-winning field goal by Jason Elam.

But some people think that the rivalry of the teams is so great that it transcends how good the teams are. That means, no matter how good each team is, the rivalry makes the teams more competitive than you would expect. So the Eagle decided to compare how well the Chiefs and Broncos did each year with who won and how close each of their games were.

The biggest cluster of games is “expected Chiefs losses” – that’s because the Broncos have had the better records since 1978, so they have been expected to win more often. That also means the smallest quadrant is expected Chiefs wins.

But interestingly the Chiefs have more unexpected wins than they do unexpected losses. Part of that is just that the Chiefs had more opportunities for upsets because they were the underdogs more frequently.

If you look closely, the Chiefs won 22 times when they were expected to lose and only lost 25 times. That means that, in games when the Broncos were better at the end of the season, the Chiefs still had nearly a 50 percent chance of beating them.

But in the games the Chiefs were expected to win, they ended up losing 13 and only winning 9. When they finished the season with the same record, they won twice and lost twice.

So the Chiefs actually have done better against the Broncos when they turned out to be a worse team by the end of the year. This lends credibility to the idea that, when it comes to the Chiefs-Broncos, the rivalry really does matter.

Nate Silver, who gained fame for predicting the past couple of presidential elections for the New York Times, has inputted the results of every pro football game ever into a new model for fivethirtyeight.com. The results show how each team has been doing over time. Among other improvements, Silver’s model includes how every team was doing during every single game of every season, not just how they ended up at the end of each year.

Below we can see how the Chiefs and Broncos have been doing compared to each other and the rest of the NFL over time. The horizontal line in the middle shows how the average NFL team is doing. So when the Chiefs or Broncos squiggle goes above the line, that means they were better than the average NFL team and when their squiggle goes below, they were doing worse.

The Chiefs peaked much earlier than the Broncos, according to the model. Compared to the rest of the NFL, the Chiefs were best on Jan. 11, 1970 – the date of Kansas City’s 23-7 victory over Minnesota in Super Bowl IV.

The Broncos, on the other hand, have been doing better overall since then. The Broncos peaked on Dec. 6, 1998 with its 35-31 win over the Chiefs to improve to 13-0. Coming off its first Super Bowl win following the 1997 season, Denver won its second straight championship in Super Bowl XXXIII.

While the Chiefs have had four or five periods of playing below-average football, the Broncos have only had one serious period of poor performance, just after 2010. And while the Broncos have been at the top of the league on four or five separate occasions, the Chiefs have only been on top of the league once since 1970 – in 2003.

Including their games as the Dallas Texans, the Chiefs won 19 out of 20 games in the 1960s but since then the Broncos have won every decade:

This story was originally published September 17, 2015 at 11:58 AM with the headline "Chiefs-Broncos games are closer than you might expect."