Matt Schaub is pretty much an average QB.
If you rank near a score of 50, according to the QBR stats (since we're supposed to be "all about the stats" around here), then you're an average QB.
For an explanation of what stats factor into a QBR ranking system, click HERE
1 Peyton Manning 84.1
2 Tom Brady 77.1
3 Colin Kaepernick 76.8
4 Matt Ryan 74.5
5 Aaron Rodgers 72.5
6 Robert Griffin III 71.4
7 Alex Smith 70.1
8 Russell Wilson 69.6
9 Drew Brees 67.9
10 Eli Manning 67.4
11 Andrew Luck 65.0
12 Ben Roethlisberger 62.8
13 Tony Romo 62.7
14 Matt Schaub 62.6
The final major step is to look at how "clutch" the situation was when creating expected points. A normal play has a clutch index of 1.0. For instance, first-and-goal from the 10-yard line in a tie game at the start of the second quarter has a clutch index of almost exactly 1.0. A more clutch situation, one late in the game when the game is close -- the same situation as above but midway through the fourth quarter, for example -- has a clutch index of about 2.0. Maximum clutch indices are about 3.0, and minimum indices are about 0.3.
These clutch index values came from an analysis of how different situations affect a game's win probability on average. One way to think of it is in terms of pressure. A clutch play is defined before the play by how close the game appears to be. Down four points with three seconds to go and facing third-and-goal from the 3-yard line -- that is a high-pressure and high-clutch index situation because the play can realistically raise the odds of winning to almost 100 percent or bring them down from about 40 percent to almost zero percent. The same situation from midfield isn't as high pressure because it's very unlikely that the team will pull out the victory. Sure, a Hail Mary can pull the game out, but if it doesn't work, the team didn't fail on that play so much as it failed before then. On third-and-goal from the 3-yard line, failure means people will be talking about that final play and what went wrong.
The clutch indices are multiplied by the quarterback's expected points on plays when the QB had a significant contribution, then divided by the sum of the clutch indices and multiplied by 100 to get a clutch-valued expected points added per 100 plays.
A Rating from 0 to 100
The final step is transforming the clutch-valued expected points rate to a number from 0 to 100. This is just a mathematical formula with no significance other than to make it easier to communicate. A value of 90 and above sounds good whether you're talking about a season, a game or just third-and-long situations; a value of four or 14 doesn't sound very good; a value of 50 is average, and that is what QBR generates for an average performance.
That being said, the top values in a season tend to be about 75 and above, whereas the top values in a game are in the upper 90s. Aaron Rodgers might have gone 31-of-36 for 366 yards, with three passing TDs, another TD running, 19 first-down conversions, and eight conversions on third or fourth down in one game -- for a single-game Total QBR of 97.2 -- but he can't keep that up all year long. Pro Bowl-level performance for a season usually means a QBR of at least 65 or 70. We don't expect to see a season with a QBR in the 90s.