I don't think sports referees/umpires are biased from the beginning, unless they have some connection to the teams that they are refereeing. I think that referees are most likely to become biased based on some action during the game. For example, if an umpire in baseball calls someone out on strikes, and the players yells at him, chances are that the next close pitch, the umpire is going to call him out again because he yelled at him. I have seen this first hand in many sports that I play, such as baseball/hockey/tennis.
I also dont think that umpires cheat regardless. Since their job is to call the game as they see it, I think they try to keep it as fair as they can, or, well, they will get fired.
Good luck on your report, hope I helped.
