A health psychologist is conducting an experiment to investigate the notion that short-term psychological distress influences the immune system. Participants in the first group watched a 30-minute film in which a carpenter and his assistant demonstrated how to build a deck for a house. Participants in the second group watched an almost identical film, except that during the last few minutes of the film, the carpenter slipped while operating an electric hand saw and cut off one of his assistant's fingers. Immediately after the film, blood samples were drawn from each subject and assessed for the number of disease-fighting white blood cells. What is the independent variable in this experiment?