There's so many different things that can influence the outcome of a match.
Recent form
Head-to-head record
Weather
Crowd size
Away team time/distance to travel
Injuries
New manager
Motivation (most teams playing Man U treat it as the biggest game of the season)
"Need-to-win" (at times of relegation and championship deciders)
Formation
Derbies
What are some others?
Recent form
Head-to-head record
Weather
Crowd size
Away team time/distance to travel
Injuries
New manager
Motivation (most teams playing Man U treat it as the biggest game of the season)
"Need-to-win" (at times of relegation and championship deciders)
Formation
Derbies
What are some others?
