Here's what OBR says:
Rule 2.00: "ORDINARY EFFORT is the effort that a fielder of average skill at a position in that league or classification of leagues should exhibit on a play, with due consideration given to the condition of the field and weather conditions.
Rule 2.00 (Ordinary Effort) Comment: This standard ... is an objective standard in regard to any particular fielder. In other words, even if a fielder makes his best effort, if that effort falls short of what an average fielder at that position in that league would have made in a situation, the official scorer should charge that fielder with an error."