A top-five team has suffered an upset loss in each of the past two weekends, shaking up the national championship race and putting everyone on notice for what might be a wild year. The Oklahoma Sooners and USC Trojans certainly haven't been eliminated from contention, but each proved that perceived invincibility is anything but in college football.

In the dominant SEC run during the past six seasons, four of the six national champions suffered at least one loss along the way, and only one of those hiccups came in September. The eventual national champion this year may be undefeated right now, but the toughest tests lie ahead. And as strong as they have looked at their best, the current Associated Press top five each has some vulnerability. We can't pinpoint when upset losses will occur, even when we know they are inevitable. What we can do is look for clues in the similarities and differences between the top teams of today and those of the recent past.

We track dozens of statistics at Football Outsiders, including a number of metrics that isolate offensive and defensive drive efficiency success. Every team has its own unique statistical profile, but we can run a test to identify teams that have more statistical similarities than not. We ran such a similarity test, comparing the current AP top five teams against every team from each of the past five years. The results don't necessarily outline a predetermined path, but the commonalities (and differences) give us some indication of what to watch for in October and November.

No. 1 Alabama Crimson Tide

Most similar stat profile: '08 USC (12-1)

Alabama fans might expect that this year's Tide compare most closely with last year's championship team since the Alabama defense has been so ruthlessly efficient in the early part of the year, just like in 2011. But the '08 Trojans are a better fit by our similarity test due to success on both sides of the ball, especially in a statistic we track called value drive percentage.

To read how Alabama, Oregon, LSU, FSU and UGA compare with similar past title contenders, you must be an ESPN Insider.

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