With the offensive analysis out of the way, it’s time to focus on the mid-year statistical review of Notre Dame’s defense. But before getting to the numbers some logistical details must be outlined.

As was the case with the offense, the individual game performances (Nevada, Michigan, Michigan State, Purdue and Washington) tell part of the story, but looking at the first half of the season as a whole tends to remove situational effects and give a better overall picture. Admittedly, this is a more facile proposition at the end of the year, but five games are enough to get a decent start.

The statistical analysis below focuses on benchmarking to adjust for the level of competition. As outlined here and here, this is important for multiple reasons. The benchmarking metric of choice is termed a performance ratio (PR) and is used to adjust the Irish defensive performance to account for the play of opposing offenses.

For a detailed discussion on performance ratios go here and/or here. Simply speaking, values greater than zero indicate the Irish are performing above the average level of their competition and values less than zero are indicative of below average performance.

The data below is presented in tabular form and a description of the column headers is provided here. All numbers were taken from the NCAA statistics website and are accurate as of October 4, 2009, i.e. these values do not incorporate the performance of Notre Dame’s opponents over the previous bye weekend.

For comparison purposes, the mid-year and year-end defensive assessments for last year can be found here and here.

Now that logistics are covered, in the words of head coach Charlie Weis, “let’s dive right in.”

What Happened First?

First down defense is always important, but even more so for the Irish.

First, the Irish defensive personnel are better suited for long down and distance situations. Even though the secondary has not performed up to expectations, it is the strength of this unit. The front seven are athletic and can—in theory—get to the passer, but lack experience and a dominant player that forces offenses to alter game plans.

Second, co-defensive coordinator Jon Tenuta’s scheme is based on bringing pressure early and often. This is most effective in longer down and distance situations that occur later in the play series, i.e. where passing is a more obvious play-call. Forcing these down and distance situations depends primarily on the quality of the defensive play on first down.

Whether this problem hasn’t been identified by the coaching staff or isn’t being appropriately corrected is irrelevant, the Irish have been anything but stellar on first down and several additional problems stem directly from this deficiency.

Against an even play split (75 passes and 75 runs) the Irish have allowed 5.8 yards per first down play. Moreover, only 39.3 percent of first down plays (59 of 150) have gone for two or fewer yards but over 48 percent (73 of 150) have gone for five or more yards.

This frequently results in second and five (or fewer) yards, a down and distance that keeps the opposing offensive playbook open and allows for an equal run/pass threat for the duration of drives.

If Tenuta and company want to improve going forward, first down is the place to start. The Irish must minimize the weaknesses in the front seven by generating down and distance situations that force obvious play-calling tendencies.

They Call It Efficiency For A Reason

Third down is a struggle, but on a short field, the defense is solid.

Defensive Efficiency

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To be fair, the Irish have faced fairly efficient offensive teams.

While they haven’t been dominant, they have performed better than their offensive competition as indicated by the 0.19 average defensive efficiency PR ranking. Third down defense is the most glaring weakness, but the primary defensive problem is actually getting to third down.

Only 44.7 percent (67 of 150) of play series have resulted in a third down and only 19.6 percent (11 of 56) of drives have been three-and-out. The poor first down defense outlined above and a penchant for surrendering big plays (see below) are largely to blame.

When the Irish defense manages to force a third down the conversion rate for opposing teams is pretty independent of the down and distance situation.

Almost 84 percent of third down plays required three or more yards to move the chains (67.2 percent required five yards or more) but an ineffective pass rush and soft coverage enabled a conversion rate of nearly 39.3 percent (22 of 56). More often than not the guilty culprit is poor tackling.

In short yardage the results are only slightly better. The Irish defense has faced 11 third and short (two or fewer yards) situations and allowed 36.4 percent (three of nine runs, one of two passes) to be converted.

In other words, poor tackling and first down execution keep the Irish defense on the field. Averaging nearly six yards per first down play has enabled 55.3 percent of play series to move the chains without a third down and made three-and-out’s the exception. But even after good first down defense poor tackling has allowed middle and long yardage third down situations to be converted at nearly the same rate as the short yardage ones.

In the red zone things are a bit different. Tighter coverage, a reduced big play threat, and determined play from the front seven have combined to stall opposing drives mostly by creating turnovers or forcing a turnover on downs.

Totally Offensive, er Defensive

It’s pretty ugly.

Total Defense

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Aside from scoring and touchdowns (the two are obviously closely linked), the Irish have performed at about the level of their competition.

However, both categories are largely slanted by the opening game shutout against Nevada. Excluding this game the Irish have allowed nearly 30 points per contest, a number that would rank 95th in the country. Most of the points (60.5 percent) have come in the first and fourth quarters. In particular the Irish have allowed 41 fourth quarter points, a testament to a lack of depth in the front seven.

Aside from scoring, the Irish rank in the bottom quarter of all FBS schools and have been very average in both production (403.2 yards per game) and efficiency (six yards per play).

This production has come against a few decent offensive teams. Nevada ranks in the top 15 in yards per play (6) and yards per game (12), Michigan ranks in the top 25 in scoring (24), and Michigan State ranks in the top 30 in yards per play (25) and yards per game (21). But none have the talent and athleticism to consistently post these kind of numbers against the Irish.

Runnin’, Runnin’, Runnin’

A known weakness has developed into a liability.

Rushing Defense

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The rushing totals and rankings are consistent with the total defensive numbers above. This doesn’t bode well for Tenuta’s “if you can’t stop the run you’ll bleed to death” mantra.

Notre Dame fields a run defense that ranks 40 or below in every category and has allowed 4.5 yards per carry (92) and nearly 140 yards per game (67) on the ground. Both are problematic but the former is particularly troubling considering the rush average increases to 5.2 yards per attempt when sacks are excluded.

The Irish also struggle on first down and in short yardage situations. Opposing offenses are average 5.3 yards per first down carry and have picked up a first down on 56.5 percent (13 of 23) of short yardage runs.

The Irish run defense has faced some teams with decent rushing production (Nevada and Michigan) and efficiency (Nevada, Michigan and Purdue) that undoubtedly amplify the numbers. But it is inconsistent play that has largely been the problem.

Michigan State and Purdue rank 70th and 56th respectively in yards per game. Against these two foes the Irish surrendered only 179 yards and 3.5 yards per carry. But the defense also struggled to stop a woeful Husky running game that entered their contest with the Irish averaging under 110 yards (94) and 3.3 yards per carry (96).

What Was Once Thought To Be A Strength

Against pedestrian competition, the secondary has struggled.

Passing Defense

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Prior to the season many felt the Irish secondary would be among the best in the country. The numbers hardly support this as the Irish rank 41st or worse in every category and virtually every pass defense metric is worse this year than last.

Teams have been very effective (263.6 yards per game) with only Nevada throwing for fewer than 240 yards. Efficiency has also been problematic as opposing quarterbacks have completed nearly 60 percent of their passes for 7.5 yards per attempt (84) and 12.4 yards per completion (87).

Additionally, Tenuta’s blitz-heavy play-calling isn’t generating sacks, as the Irish have only recorded nine on the year (one per 19.8 pass attempts).

The pass defense has to be a concern heading into the second part of the season.

The Irish have only faced one decent passing offense (Michigan State) but opponents prefer to pass (53.1 percent of play-calls) even though Notre Dame’s run defense is suspect. When pressed the pass is also preferred as over 62 percent of opponents’ first downs have been through the air (excluding first downs from penalties).

This is another indication of poor first down defense as less than prolific passing offenses maintain balance via manageable down and distance situations that create an equal run/pass threat.

Looking For Consistency

Definition: Big—or explosive—plays are rushing gains of more than 15 yards and pass completions of more than 20.

Through five games the Irish have allowed 15 big gains on the ground and 19 through the air for 901 yards (26.5 yards per play). Stated differently, 10.1 percent of plays have accounted for 44.7 percent of opponents’ total yardage. Without these big plays the defense is only allowing 3.7 yards per snap.

Projecting these values to 12 games results in 36 explosive rushes and 46 big pass plays, both more than last year and hardly consistent with good defensive play.

Without big gains the Irish are fairly respectable against the run. The 15 big runs averaged 21.3 yards per attempt and accounted for 45.7 percent of the total yards on the ground. Excluding these plays results in a respectable 2.7-yard per carry average. Even without sacks this value only increases to 3.4 yards per attempt.

Through the air, however, the story is different. The explosive pass plays are good for a 30.6-yard average and have accounted for 44.2 percent of the passing output. But excluding these plays the Irish have still allowed 4.6 yards per pass attempt and 8.5 yards per completion. Both numbers are quite high and very similar to quarterback Jimmy Clausen’s performance this season.

In many ways it is odd that the defense has allowed so many explosive plays through the air.

The Irish secondary has played mostly soft on the outside and tight in the middle. Coupled with a pressure scheme, this usually prevents the big play as the defense keeps everything in front and forces quick throws. But much of this bend-but-don’t-break approach has been undermined by poor tackling that enables big gains via yards after the catch (and after contact for that matter).

Is That The Best You Can Do?

There is little good news in the numbers above. It does appear that the Irish defense is better than advertised when benchmarked to their offensive competition, but being better than awful is hardly an achievement.

Moreover, the numbers could be far worse. The Irish have had a decided time of possession advantage in every game but their contest with Washington. This has undoubtedly reduced the production of opposing offenses and helped the defense stay fresh, particularly in the front seven where there is little quality depth. Additionally, the Nevada game appears to be more exception than rule and excluding that contest makes the numbers look much worse.

There is, however, a silver lining: this unit has yet to play to its potential and many of the problems are fairly easy to correct. Better play on first down and crisp tackling will help the defense get off the field and minimize big plays. Both negate the advantage(s) of Tenuta’s scheme and must be corrected.

With a dominant Trojan offensive line and running game coming to town, correcting them sooner rather than later will be necessary for the Irish to continue their winning ways.