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Monday, July 02, 2007

Nebraska and Defensive Efficiency - Part I

As you can probably tell, I am fascinated by the world of sports statistics. I spend many evenings banging out spreadsheets filled with stats and variables related to Nebraska football and college football as a whole. I am constantly searching for some way of quantifying success/improvement or of determining which statistics matter and which don’t.

Recently I’ve been thinking more about the defensive side of the ball. On Monday I noticed that the poster Rojo had started a thread at both HuskerPedia and the Red Sea Scrolls about Nebraska’s Pass Defense. As always, Rojo presented some great information. This post actually led to very little meaningful discussion on HuskerPedia, although I was surprised it didn’t quickly become a Pelini – Sanders – McBride- Cosgrove – Elmassian debate. Side note – HuskerPedia currently has a poll question asking who fans would rather have as their DB coach and over 70% support Marvin Sanders. That’s a lot of support for a guy who is currently out of work. But I digress.

Anyway, Rojo opened his post with this statement:
Let’s be clear: The most important thing for a defense is keeping the other guys out of the end zone.
Technically, the most important thing is winning, but I agree that keeping the other team out of the endzone goes a long way towards this. While I enjoyed Rojo’s information, I was thirsty for more. That brought me back to some data I had put together some time ago. This data related to the "Stats That Matter" analysis at the Cold, Hard Football Facts, an NFL site. One of the most interesting statistics they track is the Bendability Index. According to the site:
Bendability Index – This is the first stat that chronicles the phenomenon of the "bend-but-don't-break" defense and provides a measure of defensive efficiency. The Bendability Index is obtained by dividing a team's total yards allowed by total points allowed, yielding Yards Per Point Allowed. A team that ranks high on the Bendability Index has the defense that opponents must work hardest to score upon. This effort is more important than total defense and, in many cases, more important than scoring defense. The Bendability Index is not purely a defensive yardstick. It is, instead, a great barometer of team success. It is a function of many team-wide factors, including general defensive strength, offense and special teams proficiency, turnover differential and Red Zone defense.
This is exactly what I was looking for, a measure of defensive efficiency. In their discussion Cold, Hard Football Facts notes that this statistic is quite telling in the NFL.
“NFC North champion Chicago not only gave up the fewest points (202) in football last year, it topped the Bendability Index, too. The Bears forced opponents to march 134 yards to score the equivalent of a single touchdown. Chicago boasted more than a tough defense; they fielded a ferociously efficient defense.”

In addition, if we measure teams by the Bendability Index:
The top seven defenses made the playoffs.
9 playoff teams ranked in the Top 10 (and 10 in the Top 11).
The playoff teams ranked from No. 1 to No. 17 – the narrowest spread.
But before I got too excited, I needed to determine if the statistic carried as much weight in college football. To do this I first constructed a spreadsheet ranking NCAA teams in terms of the Bendability Index for 2006. Next I used SMQ’s methodology for determining the relevance of a particular statistic in CFB. This meant finding the winning percentage of the Top 20 teams in the Bendability Index category, as well as the winning percentage of the Bottom 20 teams in the Bendability Index.

According to SMQ this is important because:
“…the relevance of a statistic shouldn't be measured only by the relative success of teams that perform well in a given category, but also by the relative failure of those that don't.”
Next, I calculated what SMQ sees as the most relevant measure of this analysis, the relationship between the winning percentages on the high and low ends of the Bendability Index. In other words, I subtracted the winning percentage of teams in the Bottom 20 of the Bendability Index from the winning percentage of the teams in the Top 20 of the Bendability Index to determine the relative disparity between these two groups. From a statistical standpoint, the greater the level of disparity, the more relevant the particular statistic. Or as a SMQ noted:
“the ‘most important’ category, it would follow, would be the one with the best records at the top, the worst records at the bottom and, therefore, the greatest disparity."
SMQ’s analysis found the most relevant defensive statistics based on disparity to be:

*Scoring Defense: + .546
Passing Efficiency Defense: + .467
Rush Defense: + ..448
Total Defense: + .396
Third Down Efficiency Defense: + .375
Fourth Down Efficiency Defense: + .253

*SMQ did not actually calculate the relevance of scoring defense. I was able to throw it together and come up with this figure.

When I calculated the disparity margin for the Bendability Index, I was surprised to discover it was + .522. In other words, Bendability Index actually appears to be a more relevant defensive statistic in terms of winning percentage than all but scoring defense.

So, just to clarify again:
“The Bendability Index is obtained by dividing a team's total yards allowed by total points allowed, yielding Yards Per Point Allowed. A team that ranks high on the Bendability Index has the defense that opponents must work hardest to score upon.”
I like this statistic and think we have some proof that it matters in CFB. In addition, it seems to add to the statistic of scoring defense by accounting for overall defensive efficiency or the number of yards necessary for a team to put up points on a defense. In Part II of this series I will look at how the teams ranked nationally in terms of the Bendability Index. I will also examine where Nebraska fits in with regard to this statistic.