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Reassessing Northwestern's rushing attack from a statistical perspective

Who was most responsible for NU's struggles in the rushing game last season?

Jerry Lai-USA TODAY Sports

Who will start at quarterback? What's the health of Christian Jones? Who will emerge alongside Jones at the receiver position?

A great deal of attention will be dedicated to answering those three questions during training camp and non-conference play, both by the coaching staff and by the media. But while the headlines surrounding the passing game are heating up, the running game has been left simmering on the back burner.

Justin Jackson's phenomenal freshman season has instilled confidence and excitement in Northwestern's running game. But should that be the case? Despite Jackson's stellar campaign, NU's rushing game, by almost every account, was below average. The success of any running attack is predicated on the play of both the offensive line and the running backs. What do the data reveal about the play of both units in 2014?

Thankfully we have stats from SB Nation's Bill Connelly to help us answer those questions:

Offensive Line 2014 Stats

Category Adj. Line Yds Std. Downs Line Yds/carry Opportunity Rate Power Success Rate Stuff Rate
Stat 98.4 2.85 33.0% 65.5% 18.4%
Rank (out of 128) 83 81 117 81 51

Justin Jackson 2014 Stats

Jackson ranked from a list of the 176 rushers who received at least 100 carries last season.

Category Opportunity Rate Highlight Yds/Opp. Highlight Yds/Opp. * Opp.
Stat 35.9% 4.46 1.60
Rank (out of 176) 109 117 114

The most important stats for the purpose of this article are adjusted line yards, opportunity rate, highlight yards per opportunity and highlight yards per opportunity, times opportunity rate.

Adjusted line yards attempt to separate the production of a running back from his offensive line by weighing accountability based on the length of every carry. In general, the offensive line is considered to be 20 percent more responsible for lost yardage than it is for yardage gained up to 4 yards, but 50 percent less responsible for yardage gained from 5-10 yards, and not responsible for yardage past that.

Opportunity rate (Opp. Rate) is simply the percentage of carries that gain five or more yards.

Highlight yards per opportunity (Hlt Yds/Opp.) are yards credited solely to the running back (highlight yards) divided by the number of carries that go at least five yards (opportunities). This is a great measurement of running back explosiveness.

Highlight yards per opportunity, times opportunity rate (Hlt/Opp. * Opp.) is a metric Connelly uses to measure running back efficiency.

By just looking at where NU's offensive line and primary ball-carrier ranked nationally in those respective statistics, it's clear that the running game was not very good last season. But why was that? Were these woes more a product of the offensive line play or the running back play?

It's not easy to separate the production between the offensive line and running backs with statistics. But the best way to do so is to compare adjusted line yards and highlight yards per opportunity. If a team has a high adjusted line yards ranking and a low highlight yards per opportunity, its running game relies more on the offensive line than the running backs. Alternatively, if a team has a low adjusted line yards ranking and a high highlight yards per opportunity, it depends on an explosive back to succeed in the run game.

So, which category does NU fall into? Neither actually. Anything under 100 for adjusted line yards is considered bad. Northwestern finished the season at 98.4, which placed in the 65th percentile nationally. What about highlight yards per opportunity? Jackson ranked 117th, which was also in the 65th percentile. Ergo, NU was equally as bad in both areas. So those two stats didn't clear things up. What about the others?

Let's look at opportunity rate. Opportunity rate isn't an adjusted stat, so it's shared between rushers and linemen. That makes it difficult to distinguish between runner responsibility and o-line responsibility. But what if we split up the opportunity rate into two groups? One for all of Justin Jackson's carries and one for the carries of the next two leading rushers on the team, Treyvon Green and Warren Long?

Green and Long had a combined 102 carries so I treated them as "one" back and ranked them against the other 176 backs who had at least 100 carries:

Treyvon Green and Warren Long combined stats

Category Opp. Rate Hlt Yds/Opp. Hlt/Opp. * Opp.
Stat 28.4% 5.09 1.45
Rank 170 83 133

Justin Jackson stats (again)

Category Opp. Rate Hlt Yds/Opp. Hlt/Opp. * Opp.
Stat 35.9% 4.46 1.60
Rank (out of 176) 109 117 114

Oddly enough, Green and Long were collectively more "explosive" than Jackson. But their opportunity rate was abysmal, making them less efficient overall than Jackson. The difference in opportunity rates between the two groups of runners is striking.

There are two possible explanations for this. Green and Long were much worse than Jackson, or the offensive line was even worse than the numbers indicate and Jackson pulled most of the weight in the short-distance running game. In reality, both of these things are probably true, but the latter is likely more true.

While I originally said that opportunity rate is shared between the running back and the line, the o-line is generally held more accountable for opportunity rate because the line is more responsible than the runner for the first five yards of any run. Factor in the fact that Green and Long were only marginally less efficient than Jackson and that the line had a bad adjusted line yards rank, and it looks more and more like the line is the bigger problem here, not the backs.

Just judging off of explosiveness, Jackson isn't great. But that obviously doesn't make him a bad running back. Breakaway speed just isn't his strength. That he had so much success last season given his lack of explosiveness and a below average o-line speaks volumes about his running ability within those first five yards of every run. (See Josh Burton's film analysis of Jackson for further proof.) Northwestern doesn't get many rushes that go five or more yards. But that number would be even lower if it weren't for Jackson. So clearly he helps the offense in that regard. He pulls the line's weight more than they pull his.

There has been plenty of movement on the offensive line already this offseason. Geoff Mogus has moved from left guard to left tackle, and there will likely be at least three new starters: Ian Park (left guard), Brad North (center) and Shane Mertz (right guard). While he'd rather have more solidity on the line, head coach Pat Fitzgerald suggests that the starting five could continue to change throughout the season.

"We're a work in progress there right now," Fitzgerald said Tuesday. "We've got some guys that are playing pretty well, and we've got some guys that need to come on. I said earlier, I don't have a problem with playing multiple guys up there, and I still kind of feel the same way. I don't know if we've solidified a true five yet...We're not quite there yet."

For Jackson to make that second-year "jump", he needs to be running behind a better offensive line. And with more explosive backs such as Solomon Vault and Auston Anderson set to be more involved in 2015, a higher opportunity rate could translate into more explosive plays. That's why the offensive line competition and development this camp is so vital.