This Defense Isn't Good. It Probably Never Was

I hate to be the fly in the ointment. But seriously, what is this I'm hearing about this defense being great this year? It makes no sense to me.

The argument goes something like this: The defense is top 10 in yards allowed. Sure it's at the bottom of the league in points allowed but that's more a function of an offense that consistently turns the ball over. It was really great at run stopping until it lost all of the starters in the middle of the field. Lastly, and most importantly, it played substantially beter than the offense this year.

However, this is more of a function of an analysis which is focused on the wrong statistic. Yards allowed just isn't important as a tool for positive analysis. Sure, it can tell us quite a bit when the defense is giving up quite a few yards, but that's completely different. A defense which constantly takes the field at its own 20 yard line isn't going to give up as many yards as one which does at its opponent's 20. Because our offense has been consistently awful yardage doesn't meet any sort of principled standard as a barometer for our defense.

This is why defenses should be judged not in terms of a volume statistic but instead in terms of efficiency statistics. Efficiency statistics help explain the story not just of what happened, but how it happened. For example, if our offense is charging down the field at 10 yards per pass it seems obvious that it is consistently gaining first downs and giving the defense fits. Even better here is an explanation from Advanced NFL Stats about Y/A:

Yards per pass attempt is merely pass yards divided by pass attempts. So we have a relatively weak statistic (0.31) divided by an even weaker one with a negative correlation with winning (-0.17). We would expect to have a fairly meaningless result, but we don't. Passing efficiency turns out to be strongly correlated with winning (0.61). And unless having a lead in a game ‘causes’ a team’s passes to be more successful, we can safely say that passing efficiency leads to winning.

According to NY/A has a .50 correlation with winning football games

Some might argue that correlation doesn't imply causation. But it's hard to make that case here. Does winning cause passes to be more successful? It might, but there's no good explanation of why this is the case. On the other hand long passing does greatly affect a team's explanation of winning.

Is there an underlying cause both for winning and for having a passing game which has a high NY/A? Well this is up for some debate. There are teams which have a high NY/A but don't pass very much. Instead they employ a balanced attack with an effective running game. Their high NY/A can be explained by (1) the defense being forced to bring the safeties in to stop the run and (2) passing off of play action. The wins can be explained by an effective balanced offense. However we need to be careful here. Just because there may be an underlying cause in certain cases does not mean that there is one in all cases. Further we shouldn't focus on the running game in itself. There needs to be an effective deep passing game in order for the wins to come.

So, what do NY/A tell us about our defense? Below is a table of how efficient offenses have been against our defense versus how efficient they have been on average over the course of the season:

Game Defensive NY/A Opponent's Average NY/A
At NYG 5.8 6.8
At SEA 6.6 6.5
Tam 3.9 7.3
CHI 11.1 5.4
At BAL 8.8 6.4
AT CAR 5.8 6.9
NYG 6.6 6.8
AT ATL 9.7 7.3
At PHI 6.8 5.6
CLE 5.5 5.6
WAS 10.5 7.1

In Graph Format it looks like this:

It is staggering how many big plays this defense has given up over the course of the season. In only 5 games has the defense held the opposing offense to below its seasonal average in efficiency. To make matters worse when it doesn't hold a team's offense below it's average yards per attempt, it gives up yards in a big way.

In only 5 games this year has this team had a game with a Defensive NY/A of below league average.

Remember when I said above that a team with an effective running game forces the opposing defense to bring safeties down to cover the run and thus opens up the deep passing game? Well that's actually not what's happening in this case. Dallas is Top 10 in the league in opposing rush Y/A giving up only 4.1 yards/carry. (To be fair league average is 4.3 yards/carry)

This defense has shown great promise this year. It seems to perform well, and it has seemed to perform better than last year in a lot of important ways. Yet, there's still something missing from it, and it's not turnovers. The defense gives up way too many deep passes, and let's it's opponents pass all over it. They are giving up the same average NY/A per game as last year and yet they do look better.

In my opinion the corner-back heavy free agency has proven once again that this defense's most glaring needs were not at the CB or LB position, but where most of us always thought it was, along the defensive line and at the safety position.

Another user-created commentary provided by a BTB reader.

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