Freakonomics took a look at the methods by which NFL teams evaluate talent. They found two facts about talent evaluation. First, a quarterback's performance during a season has little impact on his performance in the next season. Second, the metrics teams used to determine when a quarterback will be drafted are useless in predicting quarterback success:
So the information the NFL gathers does impact evaluations. But when we turned to the second question, we found a result that may seem surprising. None of the factors that were related to where a quarterback was taken in the draft predicted future NFL performance (and we considered a variety of per-play measures across a variety of time periods in a player’s career). We did find that completion percentage in college — a factor that wasn’t related to where a quarterback was selected in the draft — did predict completion percentage in the NFL. But our analysis indicated that less than 20 percent of completion percentage in the NFL can be explained by college completion percentage.