Evaluting a Receiver's Performance

Evaluating or “grading” a player’s performance has always been a topic of debate among coaches. Many of the coaches that I have been around use the +/- system of grading a players performance in a game (giving a + for carrying out an assignment and a – for a busted assignment). I personally have never liked this system of grading because I think it lacks objectivity. If a receiver runs a perfect route but drops the ball, do you give him a plus or a minus? If you give him both, then they cancel each other out, so just don’t give him anything? What feedback does that give the athlete and what can you as a coach get from that type of grading? Or, a receiver in a game could execute his assignment on every play and never catch a pass or execute a crucial block. Therefore the possibility exist of that receiver could grade out at an 85% while a receiver that has 5 receptions for 90