This is a contiuation of an article discussing the relative importance of factors in winning NFL games. Part 1 is here.
Turnovers can be analyzed in the same manner. If we want to know how good a defense is at intercepting passes, we should look at interceptions per pass and not total interceptions. Offenses may shy away from passing the ball if a secondary is known to be excellent at generating interceptions. Their ‘total interceptions’ stat would suffer as a result of teams leaning towards the run, but their interception efficiency would tell the real story.
Fumbles are slightly different. Once the ball is on the ground, who recovers it is a very random thing. Fumble recoveries are almost purely the result of unplanned conditions, such as where players happen to be and who happens to be facing the direction of the ball. (Note: Skills like running, passing, or generating interceptions are repeatable and enduring skills which tend to carry over from year to year. Fumble recovery percentage does not.) The best measure of the fumble, therefore, is the forced fumble, which does carry over from year to year. Defenses can be good at jarring the ball loose, but who recovers the ball is random. The more often the forced fumble, the more opportunities the defense has of recovering. Forced fumble rates correlate better with wins, which bolsters its case as the stronger factor in winning.
|Def Forced Fumbles||0.41|
On the offensive side of the fumble, the opposite is true. Unforced fumbles are the repeatable, enduring skill--or I should say avoiding the unforced fumble is the skill. Therefore, the fumble per play stat is the optimum stat to measure an offense’s tendency to fumble. Keep in mind that fumbles happen on both pass and run plays.
So far I’ve ignored special teams. There’s a reason for that—they don’t really matter on a consistent basis. Of course, we can remember a critical play here, a miracle play there--they can change the outcome of a game on rare occasions. We can remember these spectacular plays because they are so exceptional. But no matter how we measure special teams in a meaningful way, the correlation coefficients are not statistically significant. That’s not to say that teams can ignore special teams. Every advantage a team can find is important, but in the long run, the importance of kickoffs, punts, return yards, and field goal percentages are all dwarfed in relation to things like running, passing, and turnovers.
There are several valid explanations for the relative unimportance of special teams. First, and perhaps most obvious, is that special teams plays are far less frequent than pass or run plays. Second, the difference between the best special teams and the worst special teams is small in comparison to the spread between the best and worst offenses and defenses. Third, special teams stats are very hard to evaluate. For example, average net punt yards negatively correlate with winning at -0.17. This means the better the team is at punting, the fewer wins it can expect. But I believe this is misleading because teams with good offenses would be able to kick more “inside the 20 yard line” punts than teams that often kick from their own 30 yard line.
One last point about special teams: Big special teams plays, such as kickoff returns for touchdowns, are rare. In the past five seasons, there has been an average of 13.6 kickoffs returned for touchdowns--by all 32 teams combined. The best test of skill vs. luck I have come across asks the question, “does it tend to be repeatable?” Kickoff or punt returns for touchdowns, or 60+ yard field goals, unfortunately, do not tend to be repeatable.
Penalty rates, however, do tend to be repeatable. Teams that have committed lots of penalties tend to continue to commit more, and vice versa. Experience, discipline, preparation, and playbook complexity probably all contribute to penalty rates. Other factors probably come into play. For example, it might be expected that weak secondaries would tend to commit more pass interference penalties in desperation to prevent deep passes.
Penalties and penalty yards correlate negatively with winning, as we’d expect. (Penalty rate is defined as penalty yards per play.)
|Team Pen Yds||-0.18|
|Team Pen Rate||-0.37|
The penalty rate stats have the stronger correlations with wins than either penalties or penalty yards.
Continue reading part 3 of this article.