Defensive Efficiency: Turnover%
This is the third post in my series discussing Defensive Efficiency Scores (D-SCORE). Here’s the first two parts if you missed them:
The last variable to explore in evaluating defensive efficiency is Turnover% – which is calculated by Turnovers/Drives allowed.
Here are the best Turnover producing defenses since 1970:
The 1984 Seattle Seahawks lead the way, having forced a turnover on almost a third of their opponents’ offensive possessions. You’ll notice that most of these teams are from the 1970’s and 1980’s. This is because turnovers declined fairly significantly during the 1990’s:
Because of the steady decline – almost 7% in 30 years – in TO%, we need to look at a defense’s TO%+ (measuring a team’s TO% to their league average):
Check out the “Diff” column on the right. This represents the difference between a defense’s Points Per Game allowed (DPPG+) and Yards Per Drive allowed (YPD+). What you’ll notice is that many of the best Turnover%+ defenses have big differences between DPPG+ and YPD+. The higher the difference, the more a defense “bends but doesn’t break”. A handful of these teams (2007 Colts, 2010 Patriots, 1993 Bills, 2004 Patriots, 1998 Seahawks) were decidedly below average in preventing yards, but were quite good at preventing points. A big part of their success came from creating turnovers. Interestingly, the 2005 Bengals sit atop this list despite being below average in DPPG+ and YPD+. They are a rare case of a team being great at taking the ball away but sub-par at everything else.
Taking the ball away is something that most elite defenses do. Here are the best DPPG+ defenses since 1970 and their corresponding TO%+
Only three of the top 25 were below average in taking the ball away. As a group, these defenses averaged a TO%+ of 121 – meaning that they created turnovers on 21% more drives than league average.
Here are some averages to consider:
What this shows is that teams with a TO%+ of 115 or greater (i.e. they take the ball away 15% more than league average) tend to allow 14% fewer points than average. Teams who take the ball away an average amount (96-104) tend to give up an average amount of points (102). Unsurprisingly, teams which are bad at creating turnovers tend to be bad at preventing points. Here it is in graphical form:
An additional effect of generating a lot of turnovers, is that the chance of a defensive touchdown increases. Between 1970-2010, there were 38,212 takeaways and 2,710 defensive TD’s scored. So about 7% of turnovers over that time period directly result in a TD. Over the course of the season, that’s probably only one extra defensive TD for an elite defense – but that one defensive TD is a potential 14 point swing on the scoreboard. Another side effect (one which is difficult to quantify) is that defenses which force lots of turnovers set their offenses up with better field position – leading to an increased likelihood of the offense scoring.
To restate an obvious point: a defense’s goal is to stop the opposing team from scoring. As long as they do that, it doesn’t really matter how many yards they give up in the process. The best way to stop the opponent from scoring is to generate a turnover. Not only does this prevent the opponent from scoring, but it can lead to points directly or indirectly. To that end, creating takeaways is the most valuable thing a defense can do to affect the margin of victory in a game.
Check back later in the week, as I put all these variables together and unveil my final Defensive Efficiency Scores.