+ is good, - is bad. Everything associated with a rating is given a +/- and a number. The number corresponds directly to a scoreboard equivalent. A player or team with a rating of +7 means that that they are one full touchdown better than average over the course of a game. Likewise, a rating of -7 means that they are a full touchdown per game worse than average.
How do I arrive there?
I havea database of play by play for all the games for a given group (NCAA or NFL) for the whole season. Based on all of this data, each combination of down, distance and line of scrimmage is assigned a point value. A 1st and 10 at your own 20 will, on average yield 1.52 points, or a little more than 1 touchdown every 5 possessions. Each play is then evaluated based on how many points were expected before the play and then after. Examples:
Oklahoma has a 3rd and 1 at their own 36. Expected points: 1.81All of these individual plays are then added up and all of the individual players and teams are awarded a cumulative score. That cumulative score is then adjusted based on the strength of the opponents unit. Colt McCoy's performance against Oklahoma began as a +9 but because he did it against the Oklahoma defense, the value was adjusted to +24. Conversely, in the controversial Washington/BYU game in week 2, BYU QB Max Hall began with a +16 but because the Washington pass defense is so weak, his score was demoted to a +6. The adjustments can be much greater for college football vs the NFL as the level of competition is much more varied at the collegiate level.
Chris Brown rushed for a 6 yard gain and a new 1st down. Expected points: 2.44
Chris Brown is awarded +.63 for getting the 1st down and moving Oklahoma closer to a TD.
Oklahoma has a 2nd and 10 at their own 24. Expected points: 1.44
Sam Bradford passes complete to Manuel Johnson for 6 yards. Expected points: 1.56
Bradford and Johnson both receive +.12 for the 6 yard gain. The gain was the same as before, but 6 yards on 3rd and 1 are a lot more valuable than on 2nd and 10, thus the points awarded are greater.
How do you account for turnovers?
There are 2 components to every turnover. The loss of opportunity for the offense and the resulting gain of opportunity for the defense. For the first half, the player who commits a turnover is deducted the same amount as if he would have failed to gain a first down on the play. However a second component is then added. Depending on where the opponent takes over, a further deduction is given. An interception 35 yards down field is essentially the same as a punt, so no additional penalty is given. However, if that interception is returned for 50 yards, the opponent has not only recovered the punting distance, but 15 additional yards as well. To calculate the change, I assume that if the drive had ended at that point, it would have given the opponent 35 yards from the line of scrimmage, but no closer than the 20 yard line. The difference in value between this spot and the value of the spot where the return ended (or 7 pts if it was returned for a TD) is deducted from the offensive play value.
How do you account for competition?
After all the game are entered, each unit gets all of their pts added up and then divided by the number of plays. A perfectly average unit will come out to 0.00 a good unit may be +0.30. Then every play's value is re-entered to account for the level of competition. A run worth +1.00 against a bad rush defense could only be worth +0.70 for the offense. However, if the rush offense is also very bad, the defense may be docked -1.30 for the play. A play will not have the same value for the offense and defense because their ratings are not the same. In fact, both teams can end the game with a positive score if both are good and the game is tight, or both teams could end with a negative score if both are bad and neither shines.