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Adjusted Stats 2009 Year in Review Part VIII

This is the eighth in a series of posts that will review the 2009 season (as well as 2008) from an adjusted stats perspective.

Tentative Publication Schedule

Part I: Which Stats Correlate Best to Winning? - 6/24/10
Part II: Drilling Down and Regression Models - 6/28/10
Part III: Passing Efficiency Formula - 6/30/10
Part IV: Conference Strengths and Pace - 7/2/10
Part V: Testing Conventional Wisdom - 7/16/10
Part VI: Team Matchups - 7/20/10
Part VII: Year-to-Year Changes - 7/22/10
Part VIII: Points per Yard Efficiencies - 7/26/10
Part IX: Data Dump - Team Rankings - 7/28/10
Part X: Data Dump - Team Reports - 7/30/10

Points per Yard Efficiencies

One of the things we’ve been keeping an eye out for is the influence of luck on a team’s performance within a season. Earlier we found that the number of fumbles forced by a defense per carry by their opponents is essentially random, or is based more on their opponents than it is on any skill of the defense. This concept of luck is occasionally implied when an analysis predicts that a team will regress to the mean in a given statistic from one season to the next. If there is no luck or random behavior associated with a statistic then we should expect performance to be highly correlated from one season to the next as roster and coaching changes would be the only major influences.

Phil Steele’s 2010 season preview presented a hypothesis regarding a certain statistic and its random nature. In his Texas preview, Steele noted that Texas scored many more points in 2009 than their yardage production would indicate should have been expected and that he expects the Longhorns to regress to the mean in 2010. But should he? At first glance it seems logical that a team with a good defense will perform better in this points per yard statistic as their defense leaves their offense with shorter fields. And if that’s the case then this efficiency is not random and there should be signs of correlation from one season to the next. But first I wanted to test my hypothesis that a strong defense was the driving factor in this statistic. The efficiency rating for each team was calculated simply by dividing their adjusted offensive points per play (no defensive or special teams scores included) by their adjusted offensive yards per play. The correlations between that calculated statistic and the team’s adjusted yards allowed per play:

Statistic 2009 Coefficient 2008 Coefficient Average
Defensive Yards per Play -0.581 -0.543 -0.562

The relationship is confirmed. A strong correlation exists between the amount of yardage surrendered by a team’s defense and the number of points their offense scores per yard gained. Not surprising, rather the strength of the correlation is logical and we would then expect that a team’s performance in points per yard efficiency should correlate from one year to the next based on defensive strength. The year-to-year correlation:

Statistic Year-to-Year Coefficient
Offensive Points per Yard 0.592

As suspected, a team’s performance in this statistic should not be expected to regress sharply to the mean because it is not driven primarily by randomness or luck.

Statistic 2009 Coefficient 2008 Coefficient Average
Offensive Yards per Play -0.430 -0.424 -0.427
Statistic Year-to-Year Coefficient
Defensive Points per Yard 0.642

Previous patterns once again hold true. Year-to-year performance is more consistent on the defensive side of the ball but expecting sharp regression to the mean isn't as logical as you may first think. Performance in points per yard efficiency is not random or pure luck, it is heavily dependent on the strength a team on the other side of the ball. With Texas in mind, then, we should expect the Longhorns to once again score more points per yard than the national average if we expect them to field another strong defense. Question marks surrounding their offense will actually be more likely to have an impact on their defensive points per yard figure than the offensive statistic.

This was the final study based on the adjusted stats I had planned. The next two postings this week will highlight some national team rankings and specific team reports and then provide links to somewhat large files containing the relevant data.