A researcher wants to know if alcohol consumption increases levels of aggression. She obtains a sample, of which, half are randomly assigned to the treatment condition and given alcohol and half are not. She then observes participants and gives them a questionnaire to measure levels of aggression. Aggression levels of the treatment group are then compared to the control group, and she finds that those who drank alcohol were indeed more aggressive than those who had not consumed alcohol. Which type of study does this describe?