Category: NFL

Updated: December 18, 2017

Last year I undertook an analysis of home field advantage in the NFL playoffs but only the wild-card games had been played when I published those results.  I’ve now included all the playoffs in the 2016-2017 NFL season and added some other findings.  The basic conclusions I reached a year ago have remained unchanged.

Overall the home team has won about two-thirds of these games by an average margin of just under six points.

Because the teams are seeded in the playoffs we should expect home teams to outperform their opponents.  The differences across the types of playoff games show the value of these higher seedings.  The margin is smallest in the “wild-card” games, since the teams in those games are more closely matched.  (The top-two teams in each Conference receive a bye in the first-round, so the wild-card games pair the three versus the six seeds, and the four versus the five.)  In the later rounds when the top seeds play, the home team’s advantage is larger, running about seven to eight points compared to three in the wild-card games.

Some of the home field bonus can be attributed to the fact that higher-seeded home teams are stronger, while some may reflect the “home-field advantage.”  For comparison, I calculated the average score for home and away teams for the 41 games played during weeks seven through nine of the 2017 season.  Home teams scored an average of 22.8 points, 2.9 more than their opponents, and won 61 percent of their games.  These figures are quite consistent with the results for wild-card teams presented above.  Seedings play a greater role in the later playoff rounds.

Two years back I estimated the point spreads in the four divisional NFL playoff games using a simple model of each team’s average margin or victory over the season based on these factors:

• net yardage from offensive and defensive plays;
• net sacks per game; and,
• net turnovers per game.

I have since added the effects of three other factors to my model for point spreads:

• net yards gained or lost during kickoff returns;
• net yards gained or lost during punt returns; and,
• net yards gained or lost due to penalities;

I have estimated the effects of these factors on each team’s average margin of victory using seasonal data for all 32 NFL teams between 2013 and 2016.

This table presents the results for the remaining eight teams in the playoffs.  The top half of the table shows the net difference between the home and away teams on each of our six factors.  For instance, Houston gained an average of 13.4 more yards per game compared to its opponents; for New England, the comparable figure is just 0.2.  That gives New England a net deficit in yardage of 0.2 – 13.4, or -13.2 as reported below.  The other figures in the top half of the table are similarly calculated.

Using my model, I can estimate the individual effects on the margin of victory (“point spread”) for each of these six factors. For instance, the effect for yardage is approximately 0.08 points per net yard gained, so the Texans’ 13.2 yard advantage on the ground compared to the Patriots is worth about 0.08 X 13.2, or 1.06 points, rounded to 1.0 in the table.  The team with the greatest advantage in terms of yardage is Dallas, which gained on net almost 28 yards more per game against its opponents than did Green Bay.  That difference is worth about 2.2 more points for the Cowboys in tomorrow’s game against the Packers.

When we add up the various effects of each of these six factors, Dallas has the greatest predicted advantage at slightly over three points compared to the Packers.  Next comes New England, whose advantage stems largely from creating more sacks and turnovers than do the Texans.  The Atlanta Falcons hold a slight advantage over the Seattle Seahawks, while the Kansas City Chiefs are predicted to lose to the Pittsburgh Steelers in their game Sunday night.

The last column on the right of the table shows the betting lines in Las Vegas for each game.  These are comparable to my predicted point spreads.  The 16 point spread for the Patriots over the Texans is outrageously high both by historical standards and by my estimates.  Atlanta is also more favored by bettors than the teams’ 2016 performances would justify.  And, despite my model’s prediction that Kansas City should lose to the Steelers in Arrowhead, bettors prefer the home team by a slight margin.

With the conclusion of week nine of the NFL season we have reached the midpoint of the 2015 regular season.  All teams have played in at least eight of their sixteen games with four teams having played nine since they have not had the bye week.  I decided to replicate last season’s analysis of the 2014 season to develop a “report card” for this year’s lineup of NFL team performances.

The 2014 model included three predictors of a team’s margin of victory: net yards gained per game, net turnovers per game, and net sacks per game.  The first two of these also help explain variations in a team’s margin of victory in 2015, but I find no separate effect so far for sacks.  Perhaps that is just because they are relatively rare events which makes their effect harder to measure across just eight or nine games.  I also included a new factor this year, net yards gained or lost due to penalties.  It turns out that penalty yardage has an effect on scoring that persists even after a team’s total yards gained or lost is factored into the equation.  The effect is about the same for yardage due to penalties as net yards from scrimmage, about seven or eight points for every hundred yard a teams outgains its opponent.

Including those factors lets us rank the 32 teams by how well their actual performance on the field compares to the performance my little model predicts.  At the top of the list is the Baltimore Ravens.  They have scored an average of three points fewer than their opponents, but that is almost five points more than their statistics would predict.  The next five overachievers all outscore their opponents by a margin or three or four points more than we would expect given yardage, turnovers, sacks, and penalties.

Two teams stand out as dramatic underachievers, the Tampa Bay Buccaneers and the Chicago Bears.  Both teams score an average of six points fewer than their statistics would predict  They are followed by the Chargers, Jets and Texans.  Of those three only the Jets are predicted to outscore their opponents by a substantial margin, nine points per game, but they have managed to win by an average margin of just under five.

 

 

As we head into the divisional playoff weekend, I thought I’d see how the eight remaining teams stack up in terms of our little model.  Here’s an accounting for the four games taking place this weekend:

The columns on the left show the regular-season differences between the home and away team on each of our three factors — yards gained, sacks, and turnovers — while the second set of columns uses the predicted point value of these differences based on our model.  For instance, the Baltimore Ravens outgained their opponents by 28.0 yards on average. That compares to a figure of 21.4 yards for the New England Patriots, for a net 6.6 yard advantage for Baltimore.  According to our model, that figure translates into an estimated 0.4 point advantage for the Ravens; the two teams are pretty much even in terms of net yardarge.

However the teams diverge on the other two measures, sacks and turnovers.  The famously tough Ravens’ defense has averaged one (net) sack more per game than the Patriots, which is worth about two points on the scoreboard.  In contrast, the Patriots hold the advantage in the turnover department, averaging 0.6 (net) turnovers per game more than the Ravens. The Ravens collected eleven interceptions and eleven fumbles in the regular season, but they gave up twelve interceptions and eight fumbles as well.  Tom Brady was intercepted just nine times while the Patriots defense collected sixteen.  The Patriots also collected nine opponents’ fumbles while losing just four themselves.  While the Patriots’ net advantage in turnovers is smaller than the Ravens advantage in sacks, it has a greater effect on the score, since turnovers were worth about five points on average in 2014 while sacks generated a bit over two points.

When we add up all three factors we find the Patriots with a minuscule predicted lead over the Ravens.  Of the four games this weekend, this is by far the closest match-up using the teams’ regular-season performance on our three factors.

Looking at the other three games, Indianapolis has the toughest road; our model makes them a nine-point underdog to Denver.  The Broncos lead on all three measures, but the biggest gap between the teams comes in the turnover category.  The Broncos collected eighteen interceptions and seven fumbles in the regular season, while giving up only five each.  The Colts figures were twelve versus sixteen for interceptions, and fourteen versus fifteen on fumbles.  The model rates the Broncos’ turnover advantage as worth about five points.

In the NFC, Green Bay’s advantages over the Dallas Cowboys look rather similar to Denver’s advantages over the Colts.  The teams were about equal in terms of yards gained, but the Packers have solid leads in sacks and turnovers.  Out in Seattle, the Seahawks outgained their opponents by over a hundred yards on average.  Comparing that difference to Carolina’s meagre seven yard advantage over its opponents predicts Seattle should score nearly a full touchdown more than Carolina. The Seahawks also gain from a lower turnover rate, but it has a much smaller effect on the point spread than does Seattle’s advantage in yards gained.

I’ve updated the little model for a team’s margin of victory using the complete 2014 regular season data from the NFL and ESPN sites.  We can use its predictions to see which teams performed better or worse than we would expect based on its net yards per game, turnovers, and sacks.

The horizontal axis shows each team’s predicted margin of victory based on the model. Teams playing the Oakland Raiders are predicted to beat them by an average of about eleven points per game.  Teams playing the Denver Broncos faced the opposite fate; the model predicts that Denver will outscore its opponents by nearly twelve points.

The vertical axis measures the difference between each team’s predicted margin of victory and its actual margin on the field.  Now the Broncos look like underachievers since they won by nearly four points less than the model predicts.  The Carolina Panthers and New York Jets had performances similar to the Broncos.

However most stunning overachiever of 2014 will not be appearing in the Playoffs.  The Kansas City Chiefs averaged a full touchdown more than our model predicts. The next best overachiever, the Dallas Cowboys, comes in a full three points lower than the Chiefs.  The model predicts that the Chiefs should have given up about 3.3 points more than they scored; in fact they outscored their opponents by an average margin of 4.0.

Here is a breakdown of how yardage, turnovers and sacks contributed to each team’s performance in 2014.  The right-hand portion of the column compared the predicted and actual performances, the same results summarized in the chart above.

 

 

I’ve decided to kick off this blog by trying to answer a question my friends and I have pondered over the years.  How many points is a turnover worth on average?  Do interceptions have a greater value than fumbles, or do they both have about the same effect?  What about other events like sacks?  How much are they worth?

One simple approach is to plot the difference in average points scored per game minus average points allowed against a team’s net number of turnovers per game.  Using data for the 2014 regular NFL season we get this graph:

The horizontal axis measures the difference between the number of “take-aways” and “give-aways” per game.  Most teams average about half a turnover in either direction, but strong teams like Green Bay create on average nearly one more turnover than they commit. The vertical axis measures the difference between a team’s average points scored versus its average points allowed.  Teams like the Raiders, Titans, and Jaguars were outscored an average of ten points per game or more, while New England outscored its opponents by about the same margin.

The graph has the expected positive relationship; teams that create more turnovers than their opponents win by larger margins.  There is a problem, though.  The best-fit “least squares” line has an implausibly large slope of 8.7 points per turnover.  Since a turnover results in zero, three, seven, or at most eight points, an average of over eight points per turnover is obviously too high.

The problem is that teams that create more turnovers than they give up are often also more skilled in other aspects of the game.  The right-hand side of the graph where teams that collect more turnovers on average are found has powers like Seattle, New England, Green Bay and Denver.  While those teams may have an advantage over their opponents in turnovers, they have strengths in many other aspects of the game as well.  So the apparent effect of turnovers also indirectly measures a team’s abilities in rushing, passing, and kicking.  Good teams do many different things well, one of which is turning the ball over less often than their opponents.

What we need is a method of “controlling” for these other team factors so we can measure the effects of a turnover more accurately.  There are obviously hundreds of football statistics we might use to predict scoring performance, but I’ve limited my attention to just three — yards gained or lost, turnovers, and quarterback sacks.  As before, I’m using the 2014 NFL team statistics through week 16, augmented by ESPN’s compilation of take-aways and give-aways.

I’ve put the statistical results in a separate “Technical Appendix;” here are the results:

A team that takes away one more turnover than it gives up scores on average 4.4 more points per game. A sack is worth about half as much.

We can also calculate how many additional yards a team would have to gain to get the same scoring advantage as averaging one more turnover per game.  That works out to

(4.4 points/turnover) / (0.067 points/yard) = 65 yards/turnover

Using the same method a sack has the equivalent effect on scoring as gaining an additional 33 yards.

While this “model” of football doesn’t include many other important features of the game like kicking, these three factors alone still account for over 80 percent of the variation in scoring.  With that in mind, let’s ask how teams have performed this season compared to the model’s predictions.  That gives us a method to identify underperforming and overperforming teams.

The Kansas City Chiefs scored over a full touchdown more per game than we would predict knowing the team’s net yardage gained, turnovers, and sacks.  The Cowboys and Patriots each averaged about four points more than our model predicts.  The most surprising result has to be the Denver Broncos.  The Broncos prowess in all three categories measured here predicts they should outscore their opponents by nearly twelve points per game rather than the 7.5 points they averaged through week 16.

Here is a detailed accounting of the effects of each of our three factors on teams’ performances through week 16.  I’ve bolded the largest and smallest effects in each category.  Seattle leads the league in net yards gained at 101, which is worth 7.2 points. Hapless Oakland has the worst net yardage which costs them an average of 4.2 points per game, and the Raiders lose about the same number of points by averaging 0.9 turnovers more than their opponents. Green Bay gains 4.4 points from its league-leading turnover difference of +1.0.  The Baltimore Ravens again show their defensive toughness leading the league at +1.8 sacks per game, worth 4.0 points on average.  Jacksonville’s inability to protect its quarterback cost the Jaguars about 3.5 points per game.