Ahealth psychologist is conducting an experiment to investigate the notion that short-term psychological distress influences the immune system. participants in the first group watch a 30-minute film in which a carpenter and his assistant demonstrate how to build a deck for a house. participants in the second group watch an almost identical film, except that during the last few minutes of the film, the carpenter slips while operating an electric hand saw and cuts off one of his assistant's fingers. immediately after the film, the researcher draws blood samples from each subject and assesses the number of disease-fighting white blood cells. what is the independent variable in this experiment?