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You know it’s going to happen, it always does…you just hope it doesn't happen to you.

Yeah, some number-nothing team is going to prance into No. 2’s home field as 21-point underdogs and win by three points. Suddenly, the national championship dream is over and the big dog is cast away to the distant shores of the Fiesta, the Cotton or the Chick-fil-A.

Though you know it’s coming, have you ever stopped to think about how it happens?

If a guy had a Ph.D. in Upset-ology—from Northwestern, Duke or Stanford—how would he explain a phenomenon that can’t be seen with the naked eye and can’t be accurately predicted but is as real as the cross-dressers at an LSU game?

You’ve got to figure that the first thing he’d do is look back at a group of 20 upsets from the BCS era and attempt to establish definite trends that connect these exceptional events.

Bleacher Report did just that, and we uncovered the ultimate checklist for coaches seeking that program defining upset.

Underdog: Fast Start/Big Run

The first building block of the upset is simple: The underdog has to strike first in a big way, or it has to have a significant run at some point in the game.

Of the 20 BCS-era upsets analyzed, 60 percent of the underdogs scored first, while 70 percent put together a 14-plus-point run during the contest.

To illustrate, think back to September 2008 when unranked Oregon State managed to shock No. 1 USC in Corvallis.

Not only did the Beavers strike first, they scored 21 unanswered points in the first half. Even though the Trojans charged back with 21 points of their own in the second half, Oregon State needed only a single score in the fourth quarter to seal the upset 27-21.

In the iconic Michigan-Appalachian State shocker from Week 1 in 2007, the No. 5 Wolverines struck first with a Mark Hart touchdown and led 14-7 at the end of the first quarter. Even though Michigan scored first, the game—and the upset—was defined by the Mountaineers’ 21 unanswered points in the second quarter.

The Wolverines never fully recovered and, despite holding App State to a pair of field goals in the second half, lost 34-32.

Favorite: Poor Rushing Performance

Perhaps the least-celebrated—but most consistent—key element of the upset is that the favored team struggles to run the ball.

Of the 20 upsets studied, a whopping 90 percent of the favorites did not have a 100-yard rusher. Beyond that, 65 percent of the favorites had fewer than 125 yards rushing as a team.

The message is clear: If an upset is going to happen, the favorite’s running game must be shut down.

Consider the case of the 2009 Sugar Bowl when No. 4 Alabama fell 31-17 to No. 6 Utah. The Tide rushed for a total of 31 yards in the game and their top rusher on the day—Glen Coffee—ran for 36 yards.

Then there was November 2012, when unranked Baylor destroyed No. 1 Kansas State 52-24. In this game, K-State—which finished the season ranked No. 32 in rushing offense—managed only 76 yards on the ground. The Wildcats’ top rusher in the upset—John Hubert—ran for 43.

Turnover Battle: Underdog Wins

What’s critical in upsets is not so much not making any mistakes, but rather, making fewer mistakes.

So, while the underdog needs to not turn the ball over, it’s more important that they turn it over less times than the favorite.

Of the 20 upsets evaluated, 16 (or 80 percent) shared the trait of the underdog winning the turnover battle. Furthermore, seven (or 35 percent) of the underdogs did not commit a single turnover.

To illustrate this in the extreme, look back to October 2007 when unranked Stanford took down No. 1 USC in the Coliseum. This was a 24-23 upset fueled by the Trojans’ five turnovers (one fumble lost and four interceptions) versus Stanford’s one interception.

The Cardinal scored its winning touchdown on a drive that was set up by the third of USC quarterback’s John David Booty’s four picks on the day.

Proving that you can score an upset while making several mistakes, Boise State’s thrilling Fiesta Bowl win over Oklahoma in 2007 featured seven combined turnovers.

In this case, the heavily favored Sooners registered four turnovers (one fumble lost and three picks), while the Broncos recorded three (two fumbles lost and one interception).

Although Boise State was far from perfect, winning the turnover battle prevented the one critical mistake that would have changed the outcome.

Speaking of mistakes, turnovers and upsets, take a look at what Virginia Tech offensive coordinator Scot Loeffler had to say, according to an Associated Press report by Kurt Voigt via The Columbus Dispatch, after the No. 16 Hokies were shocked by unranked Duke, 13-10, earlier this season.

"There's a fine line between winning and losing, and we've been walking that line...Today, our kids learned how important the details are—lining up correctly, not jumping offsides, the four turnovers. All these things will come back to bite you," Loeffler said.

Narrow Margin of Victory

The statistics make this point crystal clear: Giant killers are made in nail biters.

Of the 20 upsets studied, 85 percent were decided by fewer than seven points. The overall average margin of victory was 5.8 points.

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Narrow shockers that come to mind are Ole Miss’ 31-30 triumph over No. 4 Florida in 2008, unranked Syracuse’s 38-35 upset of No. 18 Louisville in 2007 and FCS Jacksonville State’s 49-48 double-overtime shocker over Ole Miss in 2010.

Favorite: Missed Field Goals

Nine of the 20 upsets examined—or 45 percent—included a missed field-goal attempt by the favored team. Of these, four included two missed field goals.

Again, the numbers send a clear message: If you have a struggling field-goal kicker, you are more likely to suffer an upset.

Examples include No. 9 West Virginia’s shocking loss to Pitt in 2007. This was the year the Mountaineers needed a win over the 4-7 Panthers to advance to the national title game. Instead they fell 13-9 at home.

Included in the four-point loss were two missed field goals by Patrick McAfee, both in the first quarter. The first was a 20-yard chip shot and the second a 32-yarder.

Next up, think back to Texas Tech’s 41-38 shocker over No. 3 Oklahoma in October of 2011, the upset that broke the Sooners' 39-game home winning streak. Oklahoma kicker Michael Hunnicutt went 1-of-3 on field goals that night, including a 39-yard miss in the second quarter and a 28-yard miss with fewer than three minutes remaining in the game.

To illustrate the flip side of this, consider this season's game between No. 1 Alabama and unranked Mississippi State. The upset-minded Bulldogs had a shot to tie up the game early in the second quarter but missed a 23-yard field goal attempt. Instead of the score sitting at 3-3, Alabama scored two drives later and went into the half with a 10-3 lead.

The Tide went on to win the game 20-7.

According to Cliff Kirkpatrick of the Montgomery Advertiser, Nick Saban acknowledged the significance of the missed chip shot in his halftime interview. "We got lucky on one drive and got out of it, down here where they missed a field goal," Saban said.

Drama

An upset game is going to be a lot more like a group of junior high school girls at the mall than a night out on the town with your CPA. In other words, it’s not going to be dull.

With all of the twists and turns of a blockbuster thriller, the upset is reality TV gone mad.

Think back to 2005 when unranked TCU had No. 7 Oklahoma on the ropes in Norman. The Sooners were down 17-10 and driving the ball with fewer than two minutes to go when quarterback Paul Thompson got sacked by TCU’s Jared Kesler. Thompson fumbled the ball, the Horned Frogs recovered and the upset was sealed.

Then there was 2011 when No. 2 Oklahoma State’s kicker Quinn Sharp missed a 37-yarder at Iowa State to go to the national championship. Instead of facing LSU for all the marbles, the Cowboys lost 37-31 to the Cyclones in double-overtime and finished No. 3 in the BCS.

Finally, what about the 2007 game when No. 1 LSU fell to Kentucky in triple-overtime? This thriller ended with the Wildcats stuffing the Tigers on a 4th-and-2 from the Kentucky 17-yard line, icing the nation’s top team 43-37.

Statistics courtesy of ESPN, Sports Reference/College Football and College Football Statistics.