OK, here's some info...
3) How Do We Grade?
Each grade given is between +2 and -2, with 0.5 increments and an average of 0. A positive intervention in the game rates a positive grading and vice-versa. Very (very) few performances draw a +/-2 rating. In fact, the distribution of non-zero grades is like this:
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+2.00.01percent
+1.50.3percent
+1.016percent
+0.537percent (unbalanced because of the way WRs and HBs are rated)
-0.524percent
-1.022percent
-1.50.5percent
-2.00.01percent
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The grading takes into account many things and effectively brings “intelligence” to raw statistics.
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For example, a raw stat might tell you a tackle conceded a sack. However, how long did he protect the QB for before he gave it up? Additionally, when did he give it up? If it was within the last two minutes on a potentially game-tying drive, it may be rather more important than when his team is running out the clock in a 30-point blowout.
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The average grade, or what we would typically expect of the average player, is therefore defined as zero. In reality, the vast majority of grades on each individual play are zero and what we are grading are the exceptions to this.
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A seal block on the backside of a play, for example, is something that it is reasonable to expect to be completed successfully. Consequently, it receives a zero grade, whereas the differentiation between a good and poor block is a heavy downgrade for a failed seal block to the backside of a running play.