October 17, 2012 - by Ed Feng
This is a guest post by Ed Feng, founder of The Power Rank, a sports analytics site specializing in college football. If you’re interested in guest posting on TeamRankings, email us your post and we’ll consider it.
A defense’s job is preventing the opposing offense from scoring. So it makes sense that scoring defense, or points allowed per game, is the ultimate measure of a defense, right?
No so fast. Scoring defense can be a misleading statistic.
First, this number includes points allowed by the offense, such as an interception returned for a touchdown, and special teams. Second, a defense can give up fewer points through timely fumble recoveries and interceptions.
Surely, we can give defenses credit for these takeaways, right? There’s just one problem …
If you take a look at the data, you’ll find that turnovers forced in the first 6 games of the college football season only explain 3% of the variance in turnovers forced in the rest of the season (the correlation coefficient is 0.173). Translated: Early season forced turnovers is an incredibly weak predictor of late season turnovers.
Why? There’s an incredible amount of randomness in turnovers.
Sure, some turnovers result from a big hit from the middle linebacker. But sometimes the offense stumbles on its own.
Last year, Notre Dame quarterback Tommy Rees just dropped the ball on a critical drive against Michigan. Notre Dame lost a thrilling game in the last two minutes.
In the Fiesta Bowl, Stanford quarterback Andrew Luck had a bad exchange with fullback Geoff Meinken near their own goal line. Oklahoma State kicked a field goal off this turnover, and Stanford lost in overtime.
To find an overrated defense, we use a different measure of a defense: yards allowed per play. This gauges its ability to stop the opposing offense from driving the ball. Turnovers have little effect on this measure.
At The Power Rank, we take raw yards per play from each game and adjust it for strength of schedule to rank defenses. The rating gives an expected yards allowed per play against an average FBS team.
Let’s look at 3 overrated defenses using these rankings.
The Irish defense isn’t bad. In fact, it’s a very good defense, ranked 13th when we adjust yards per play for schedule strength. However, Notre Dame ranks 2nd in the nation in scoring defense behind Alabama. They’re not that good. Michigan threw 5 interceptions when they visited South Bend this year.
The Broncos are 12th in the nation in scoring defense, giving up 14.7 points per game. However, they are also 2nd in the nation with 19 takeaways. Boise State has sent many defensive players to the NFL in a recent years. While this doesn’t show in scoring defense, our defense rankings place them 36th in the nation.
Head coach Gary Patterson has always been known for his stingy defenses. And giving up 14.5 points per game for 11th in the nation seems to confirm this. However, TCU leads the nation with 20 takeaways. They beat SMU on the charity of 6 turnovers when their own offense only mustered 2.08 yards per play. Our defense rankings have them at 78th due to a weak schedule that includes Kansas, SMU and Grambling State.
Team Rankings has lots of awesome resources for college football. For example, you can view stats on college football defensive takeaways to start hunting for overrated defenses. The top teams on this list might be overrated; you can check out the defensive rankings at The Power Rank to see what my approaches think.
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