# Defensive Plays Against

After last week’s fumble posts, I realized that no team-level analysis would be complete without equalizing for number of plays.  We saw that the distribution in forced fumbles (team totals) over the past ten years does not appear to be random and is certainly not normally distributed.

However, it’s possible that the teams forcing the most fumbles (Chicago) simply had more opportunities.  Basically, is there a “natural” fumble rate?  If so, then the difference in forced fumbles could be largely dependent on the number of defensive plays run.

In compiling the data for this problem, I noticed something interesting, which I’ve touched on previously (very lightly).  Specifically:

There is a surprising lack of variability in # of plays run from team to team.

In my post about potential injuries and Chip Kelly’s offense, the main takeaway was that even the “fastest-pace” offenses do not, on average, run a lot more plays than “slow-paced” offenses.  Today we are looking at # of defensive plays, but since every defensive play is also an offensive play (for the other team), the higher level distribution should be the similar.

Here are the average defensive plays run per game over the past 10 years (for each team, ordered highest to lowest):

The Eagles rank 10th, with an average of 63.51 defensive plays per game.  However, the key is to look at the Max and Min of the table above.  Cleveland has faced the most plays, just under 65 per game.  Pittsburgh has faced the least, just under 60 per game.

So the long-term difference comes out to an average of just 5 plays per game.

Put differently, over the past 10 years, the defense that ran the MOST plays faced just 50 more plays than the defense that faced the fewest.  Note that this is an approximation since there are presumably rounding differences due to the use of averages rather than # of plays (only data available).

Going back to last week, it means that the variability in the number of forced fumbles for each team is likely NOT a function of # of plays (I say likely because the # of plays data does not include Special Teams, which could potentially skew the numbers, though I think that’s extremely unlikely.)  The correlation between average plays against and forced fumbles is -.03; so no connection.

While that’s not a huge surprise, the overall lack of variability in the # of plays data is.  Given how many different factors there are in every football game that affect the number of plays run, I expected to see a somewhat random but certainly WIDER distribution.

Perhaps it’s simply a result of the sample used (a big one, 10 years).  Over that length of time, teams are likely to cycle between “good” and “bad”, which may help the overall numbers even out.

Also, defenses have much less control over the number of plays they face than offenses have over the number of plays they run.  We can assume that “good” defenses will tend to face fewer plays than “bad” defenses, but that’s about as far as we can go.

Let’s look at individual season numbers.

Perhaps most importantly, the entire NFL is trending upward in terms of number of plays per game.  Here is a chart showing defensive plays per game over the past 10 years.  Remember that this is equivalent to offensive plays per game.

If you look at the Y-Axis, you can see that the overall difference isn’t huge (the increase from 2003 to 2012 is just 1.57 plays per game.) However, the trend is unmistakable, with the past 4 seasons showing the most severe increases.

To this end, Chip Kelly’s offense (presumed to emphasize # of plays) is not revolutionary; it’s EVOLUTIONARY.  Tomorrow, I’ll take a detailed look at offensive plays run (same overall data, but different team data and we can draw more from it due to the “control” of the offense).  The key point here, though, is that Chip Kelly’s offense appears to be a natural extension of what we are already seeing in the league.  We have not, of course, actually seen what Kelly’s offense will look like; but it’s safe to say his “plan” fits firmly within the long-term NFL offensive progression.

Let’s take a quick look at some individual defensive play stats.

Over the past ten years:

- The 2010 Titans faced the most offensive plays, with an average of 71.2 per game.

- The 2004 Pittsburgh Steelers faced the fewest plays per game, with 55.6.

- The long-term, league-wide average is 62.9 defensive plays per game.

- The 2006 Eagles faced 65.8 plays per game, the most for the team over the past ten years.  The 2007 Eagles faced the fewest; 61.2.

- There is, not surprisingly, a positive correlation between defensive plays per game and points against, with a value of .28.

As I said above, tomorrow I’ll do a deep dive into OFFENSIVE plays per game, which should give us some idea of what we can expect from Chip Kelly.   I looked at defensive plays today since I had to address the # of Plays effect on forced fumbles (no effect).