I believe the answer is: naturalistic observation
naturalistic observation refers to a research method that conducted by observing the subjects in their natural environment, without adding any controlled variables. If we use spectators of a game as subjects, their natural environment would be the stadium when a match is occurring.
After many media reports of aggressive behavior by parents and other adult spectators in the stands at Canadian youth hockey games, psychologists used a method called naturalistic observation to study spectator behavior. That is called naturalistic observation.